make pkgsStatic composable by not overriding all of hostPlatform

[Qyriad]

pkgsStatic was keeping hostPlatform.parsed, but all other configured parts of hostPlatform (AKA crossSystem) were discarded. This fixes that, making pkgsStatic work like pkgsLLVM, pkgsZig, etc.

Fixes #434755.

I'm not sure what of pkgsStatic.* or pkgsLLVM.* Hydra builds, so I've targeted staging instead of master.

cc @emilazy, @wolfgangwalther; you (reasonably) are against nesting the pkgs variants selectors in the first place, but since the fix is trivial I figure you can take a look

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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[wolfgangwalther]

you (reasonably) are against nesting the pkgs variants selectors in the first place, but since the fix is trivial I figure you can take a look

I can assure you, none of this is trivial. Your change leads to this:

nix-repl> pkgsLLVM.pkgsStatic.stdenv.hostPlatform.config
"x86_64-unknown-linux-musl"

nix-repl> pkgsLLVM.pkgsStatic.stdenv.hostPlatform.libc
"glibc"

Nope, this is not going to work. As @emilazy pointed out, if you're interested in that, you should look at #380342 and its predecessor #303849 - and some more linked PRs in there.

Beware of getting too deep into the rabbit-hole, though...

[emilazy]

FWIW I do want to fix the lack of input vs. output separation in platform definitions that would make this kind of operation possible to do in a way that makes sense. (I just think the package variant sets are a dubious interface to begin with.)

Unfortunately it is hard.

[Qyriad]

Yeeeep. Well, I figured it was worth a try to make it mildly-less-broken over a Proper Fix, but alas 😔