buildRustCrate: add host platform to rlib hash suffix

[lopsided98]

Motivation for this change

If a crate depends on both the host and build versions of a crate when cross-compiling, both versions of the crate have the same rlib name, and they clobber each other in the deps directory. This PR adds the host platform target triple to the hash, so the crates end up with unique hashes.

Is there a reason ___ is used to separate the components?

cc @Ericson2314 @andir

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[Ericson2314]

Is there a reason ___ is used to separate the components?

I bet someone was being cautious about pre-image attacks. Not sure if this is necessary or even sufficient, but I suppose we can leave it for now. Maybe if you want and a TODO asking someone to explain.

[lopsided98]

This breaks buildRustCrateTests because they check the specific hash value. We should probably just allow any hash, but that will require rewriting the test infrastructure a bit.

[Ericson2314]

Yeah I would just focus on testing that things that shouldn't be equal indeed are. Nix makes it very easy to make it deterministic, but leaves it to the other to make the equivalence class sufficiently fine-grained.

[Ericson2314]

Oh I forgot about the tests, I guess I'll go fix myself.

[Ericson2314]

Fixed tests in #105379.

[andir]

Ugh, the commit message doesn't cary any of the detail required to judge the change. We only have that in this PR.. This will make it harder to figure out why we did something. In my initial refactoring the git history was the most valuable asset we had on this :/

Regardless of that I think this is a good change. Thanks @lopsided98!