xonsh: add pip to test dependency and other changes

[tjni]

Description of changes

We removed pip from implicitly being in the environment in #248866 and xonsh has stopped building on staging-next. This PR does the following:

1. Add pip explicitly to nativeCheckInputs. Two tests depend on this.
2. Switch to pyproject build since that's more modern and upstream uses it already.
3. Add setuptools and wheel to nativeBuildInputs, which is what pyproject.toml asks for.
4. Change sha256 to hash opportunistically.
5. Prefix LC_ALL attribute with env to be more explicit.

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.

[natsukium]

The diff looks fine.