Add flake --override-input

[andreabedini]

This patch adds the --override-input flags as present in the nix CLI, closing #144.

A more complete implementation would have been adding MixFlakeOptions to MyArgs parents but I am not too familiar with the codebase and I opted for the simpler solution of adding the single flag.

I allowed myself to readjust the flake adding a derivation based on clangStdenv, but I can remove that commit from the PR if undesidered.

Thank you!

[Mic92]

@mergify queue

[mergify[bot]]

queue

✅ The pull request has been merged automatically

The pull request has been merged automatically at *f88571cfc9132e8f2768aa41d57f5f471941d4b6*

[Mic92]

Thanks!