feature: mutiple `--flake` args

[nrdxp]

Given that a flake can export buildable derivations in multiple places (packages, devShells, etc), it would be much simpler to simply eval them all at once by passing --flake multiple times than to expect the user to write some Nix function to bring them all together under a single heirarchy.

[Mic92]

This makes sense.

[adisbladis]

I think this makes general sense. In the same spirit you should probably be able to pass many files (while not using flakes).