Motivation for this change

This allows, eg, patches that require extraSrcs to be applied to Go packages. It's similar to overridePythonAttrs, and to the overrideRustAttrs function I propose in #46842.

Example:

hub.overrideGoAttrs (oldAttrs: {
  patches = [
    (fetchpatch {
      url = https://github.com/alyssais/hub/commit/e23a9027669e179a93437ff731b0e43691bb272d.patch;
      sha256 = "19i3sy06vqdmrqvngq8wxnwksa9h65kc4bwjmsn0pwj71asvvhp3";
    })
  ];
  extraSrcs = [
    {
      goPackagePath = "github.com/keybase/go-keychain";
      src = fetchFromGitHub {
        owner = "keybase";
        repo = "go-keychain";
        rev = "15d3657f24fc7e290714ab795239c5bb33b091ea";
        sha256 = "1357drs3pdkz92whdlrh8f6k905rb5dshvq4amajn393vwgpm2bw";
      };
    }
  ];
})

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.