DOCS: document Nix Build Passthrough flags

[ProducerMatt]

I wasn't aware until recently that nix build flags were accepted, as added in this PR: https://github.com/nix-community/nh/pull/311

It would be nice to have them mentioned in the man page, at least as a quick list of accepted flags so they show up in a grep.

If you'd like me to add them to the docs by hand, I'd be happy to. But I assumed the "correct" way to do this is making the list from the struct programmatically, and I'm not familiar enough with Rust to know how.

[NotAShelf]

I'll add those to the README and the manpages when U have some free time.

A PR would be welcome if you'd like to take a look, the manpages are created in xtask/src//man.rs using clap-mangen. Should be easy to modify

[ProducerMatt]

Hi @NotAShelf! I just took a look at adding it. It seems like the right place to include the pass through args would be when turning these doc comments into messages: https://github.com/nix-community/nh/blob/c2159253e3139c2734369393b98a33a894ceedbb/src/interface.rs#L636

But I'm not sure what's the right way to do that. Two ideas:

1. I could add a plaintext token like #passthroughargs to the comment that would get interpolated into a list of passthrough args
2. I could add a special case in man.rs that detects if the argument is extra-args and if so, appends the list to the description.

Let me know if I'm on the right track, I'm leaning towards option 1.

By the way, I just looked at your profile and was blown away by all your contributions, you're a beast 💪