I think there's a bit of confusion caused by equating Nix "derivations" with "packages" of traditional package managers.
Nix mainly concerns itself with derivations [1]. They're build recipes for creating binary artifacts that are meant to be consumed by the Nix daemon. The Nix daemon instantiates derivations by building the artifact and storing it to a store path under /nix/store. Store paths are unique to each derivation.
When people say Nix is reproducible, they mean that derivations are reproducible [2]. This is because anything that might cause the build to change is captured as inputs to the derivation. Every input is explicitly specified by the author of the derivation. This means that when a dependency gets updated, the resulting derivation and store path would change. The new derivation might fail to build, but the old one would still continue to build regardless of how much time has passed since it was first built. So if a latest package in Nixpkgs is broken, you can always go back to a known good commit to get a working derivation while waiting for the package maintainer to fix it [3].
Traditional package managers don't have a concept of a derivation. Instead, they have packages. Those packages have no reproducibility whatsoever. Even if they built successfully in the past, they might not build today. That's because a traditional package is only identified by its name and version, as opposed to a Nix derivation which is identified by its content (= the build recipe) [4]. Traditional package managers see two incompatible builds with the same name and version as the same package, replaceable with each other. Worse, most package managers don't require versions to be specified as part of dependencies. Whether a package builds or not is then dependent on the current state of the central package repository. Again, this isn't the case with Nix derivations.
[1]: Internally, Nix doesn't even have the concept of a package. A package is a concept that we humans use to group related derivations together.
[2]: To be clear, derivations aren't bit-by-bit reproducible. For example, CPU caches would be observable during builds because in general, process sandboxes don't prevent hardware information leakage. However, it's reproducible in a practical sense because people would have to go out of their way to make software builds dependent on things like CPU state. People might do that as a joke, but not for any serious reason.
[3]: Ideally, tests and reviews should catch any breakage but sometimes it happens. Hence the rolling release branch is marked "unstable." Fortunately, it's also easy to apply fixes locally before they're available in Nixpkgs because Nix makes it straightforward to create a custom derivation by extending existing ones.
[4]: Not to be confused with content addressed derivations, which identifies derivations by the resulting binary artifact.
for 2 there is sandbox from facebook to isolate tests (and builds) from cpu non determinism. i have raised ticket on nix. so really it is just another derivation sandbox.
Nix mainly concerns itself with derivations [1]. They're build recipes for creating binary artifacts that are meant to be consumed by the Nix daemon. The Nix daemon instantiates derivations by building the artifact and storing it to a store path under /nix/store. Store paths are unique to each derivation.
When people say Nix is reproducible, they mean that derivations are reproducible [2]. This is because anything that might cause the build to change is captured as inputs to the derivation. Every input is explicitly specified by the author of the derivation. This means that when a dependency gets updated, the resulting derivation and store path would change. The new derivation might fail to build, but the old one would still continue to build regardless of how much time has passed since it was first built. So if a latest package in Nixpkgs is broken, you can always go back to a known good commit to get a working derivation while waiting for the package maintainer to fix it [3].
Traditional package managers don't have a concept of a derivation. Instead, they have packages. Those packages have no reproducibility whatsoever. Even if they built successfully in the past, they might not build today. That's because a traditional package is only identified by its name and version, as opposed to a Nix derivation which is identified by its content (= the build recipe) [4]. Traditional package managers see two incompatible builds with the same name and version as the same package, replaceable with each other. Worse, most package managers don't require versions to be specified as part of dependencies. Whether a package builds or not is then dependent on the current state of the central package repository. Again, this isn't the case with Nix derivations.
[1]: Internally, Nix doesn't even have the concept of a package. A package is a concept that we humans use to group related derivations together.
[2]: To be clear, derivations aren't bit-by-bit reproducible. For example, CPU caches would be observable during builds because in general, process sandboxes don't prevent hardware information leakage. However, it's reproducible in a practical sense because people would have to go out of their way to make software builds dependent on things like CPU state. People might do that as a joke, but not for any serious reason.
[3]: Ideally, tests and reviews should catch any breakage but sometimes it happens. Hence the rolling release branch is marked "unstable." Fortunately, it's also easy to apply fixes locally before they're available in Nixpkgs because Nix makes it straightforward to create a custom derivation by extending existing ones.
[4]: Not to be confused with content addressed derivations, which identifies derivations by the resulting binary artifact.