lib/systems: revert #221707 if stdenv.hostPlatform.isPower

[ghost]

Description of changes

#221707 broke the build on powerpc64 by creating circular outpath dependencies.

This commit fixes the breakage on powerpc by reverting it if stdenv.hostPlatform.isPower.

Closes #224694

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[K900]

Maybe we should just keep this unconditionally and add a note about why it's important? Also, do you know why the built kernel includes the sources path at all? That sounds like a reproducibility nightmare for more conventional distros...

[ghost]

Maybe we should just keep this unconditionally

The conditional fixes #224694

Also, do you know why the built kernel includes the sources path at all?

It's normal. The problem is that #221707 changed us from building source that was unpacked into $NIX_BUILD_TOP (which is always /build/ for sandboxed builds, and is outside of /nix/store so it doesn't generate references) to building from source that is already unpacked into /nix/store.

It's pretty weird to unpack the source into /nix/store and build from there. That's not how the standard builder works.

That sounds like a reproducibility nightmare for more conventional distros...

Modern conventional distros build in a sandbox just like we do, and their source is unpacked into exactly the same path within that sandbox for every build. Basically inside the sandbox, the path to the source is always /build/source or something like that.

[ghost]

This PR still doesnt fix pkgsCross.powernv.linux.

Current best hope is #245449