[staging-next] gnugrep: Fix build on Musl

[dasJ]

Description of changes

• https://hydra.nixos.org/build/221679820
• [staging] gnugrep: 3.7 -> 3.11 #228714 (comment)

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/)
• 23.05 Release Notes (or backporting 22.11 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.

[dasJ]

This should work but is a draft because I currently don't have the build capacity to test this.
I might have more capacity near the end of this week.
It's still building for me.
PTAL

[vcunat]

This way it rebuilds on all platforms. We could instead use if .isMusl then foo else null;

[dasJ]

Fixed. I have successfully built pkgsStatic.gnugrep with this

[wegank]

@ofborg build pkgsStatic.gnugrep

[vcunat]

Rebuild fixed. I can now build pkgsStatic.gnugrep pkgsMusl.gnugrep

[vcunat]

(ofBorg got the rebuild labels wrong; most likely confused by the force-push)