treewide: Dont use obsolete platform aliases

[Ericson2314]

Motivation for this change

Should be the less controversial part of #45717. Doesn't formally deprecate anything, but rewrites usages of platform-related things I'd like to deprecate, analogous for #44423 for configureFlags.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-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)
• Fits CONTRIBUTING.md.

---

[FRidh]

It's a pity always having to add something like hostPlatform when you don't know/use/care about cross-compilation, but I think it is a small cost for having such feature.

[Ericson2314]

@FRidh yeah I'm going to try to make most of these ones be stdenv.is* over time.

[dezgeg]

I definitely agree in having stdenv.fooPlatform is better in place of a top-level fooPlatform reference. The reason being that say, a package foo having:

{ stdenv, hostPlatform, /* whatever */ } :
stdenv.mkDerivation {
   /* whatever that uses hostPlatform */
}

suggests that one might be able to use .override like this:

# WRONG CODE!
foo.override { hostPlatform = /* whatever */; }

But as far as I know the result of that wouldn't do the right thing. (stdenv would still have an unchanged hostPlatform).

[dezgeg]

But for the rest... I agree with this:

It's a pity always having to add something like hostPlatform when you don't know/use/care about cross-compilation,

Thus I would prefer things in this order: stdenv.isFoo > stdenv.system == "foo-bar" > stdenv.hostPlatform.system == "foo-bar".

[Ericson2314]

I'd counter that inspecting system at all is a pretty low-level operation, so full explicitness is warranted.

[dezgeg]

No, sometimes you really need an specific platform to choose from (for instance, when downloading for some prebuilt binaries for different architectures).

[vcunat]

Yes, this breaks stuff. Have you noticed the windows builds on Hydra? Before:

$ nix eval -f pkgs/top-level/release-cross.nix crossMingw32.boehmgc.x86_64-linux.system
"x86_64-linux"

After:

$ nix eval -f pkgs/top-level/release-cross.nix crossMingw32.boehmgc.x86_64-linux.system
"i686-windows"

[vcunat]

I also wonder why Borg detected some aarch64 rebuilds.