devenv: add build subcommand

[bobvanderlinden]

As discussed in https://discord.com/channels/1036369714731036712/1100034303913635860

This adds a devenv build command. It allows building derivations inside a devenv config. For instance:

$ devenv build languages.python.package

I explicitly did not pass through any options to nix build, as I wasn't sure of the use-cases and the future compatibility. We might want to explicitly add those once the use-case arises.

My use-case for this is to wrap some of the tests of my project into a Nix build, but I didn't want to migrate the project from using devenv to using devenv within flakes.

[domenkozar]

This is now part of #745

❯ devenv build languages.python.package
• Built:
• /nix/store/1r6n7v2wam7gkr18gxccpg7p5ywgw551-python3-3.10.12