Add an easy way to use packages inside run_shell

[fionera]

Is your feature request related to a problem? Please describe.
As a developer I want to invoke external buildsystems/tools that may do execs themselves. For this I would need a proper PATH with all nixpkgs I want configured.

Describe the solution you'd like
Something like run_shell but with another argument where either one label to a nix_shell_env (bad name i know) which contains all packages, or the list of packages are supplied and fitted into the run_shell environment. Bonus points if include paths etc are correct set

Describe alternatives you've considered
Hacking into nix-shell implementation to get a shell file I can use for entering. Doing it manually by including all packages manually, try rules_sh (sadly won't work in my case because of bzlmod dependency trees).

Additional context
Basically I want to use rules_nixpkgs as bazel sandbox provider

[aherrmann]

Link to corresponding Bazel Community Slack thread.