writeScriptBin, writeShellScriptBin: add meta.mainProgram automatically

[ShamrockLee]

Description of changes

Specify meta.mainProgram automatically for derivations build with writeScriptBin and writeShellScriptBin.

The script $out/bin/$name is the sole executable in the package. It makes perfect sense to specify meta.mainProgram with name.

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.

[figsoda]

but .. why?

[ShamrockLee]

@figsoda Some utilities such as NixOS/bundlers would try look for meta.mainProgram.

[figsoda]

Doesn't nix run already work since it uses name? same for bundlers it seems

[ShamrockLee]

But it may guess wrong, especially when the name contains -[0-9].

❯ nix run path:.#hello-4y
error: unable to execute '/nix/store/8hwh0frz9yf3gblx3abh1364c8xff5r6-hello-4y/bin/hello': No such file or directory

I admit that I forgot about the default-to-name behavior, and the above is an edge case. Not sure if such cases are significant enough to be considered by the builder. But it's also trivial to add and doesn't break anything.

[figsoda]

maybe this should be part of writeTextFile instead, but lgtm

[figsoda]

#234454