Rename #! nix-shell to #! nix

[fricklerhandwerk]

Of the many things nix-shell does, "entering an environment" and "wrapping an interpreted script" have absolutely nothing to do with each other.

In particular, the shebang feature is not limited to shell scripts.

It would greatly ease telling a story around Nix features (and making it discoverable) if independent features would have distinguishable names.

Alternatives

I imagine making it work for nix would require doing something involved to the top-level nix command.

Therefore, a cheap alternative would be to introduce a dedicated alias such as nix-interpreter or nix-script.

[thufschmitt]

Linked: #5189

[iFreilicht]

This was fixed by #8327, right?