Motivation for this change

Execute in subshell so that the cwd returns back
to the original directory. This is important as
it enables pythonImportsCheck to work with checkPhase

pretty much you had to do doCheck = false; or else the checkPhase (or setuptoolsCheckPhase) would run and burn because setup.py (or test paths) were no longer available

good example is: #80444 (comment)

Tested that:

• fails when an import isn't available
• succeeds when it is
• is able to be ran with a checkPhase

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.