Feature request: allow overriding flake inputs

[Rhys-T]

I'm trying to transition my NUR repo from nix-build-uncached to nix-fast-build, and need to be able to build against multiple versions of Nixpkgs like my current setup does. I'm also going to be (ab)using flake inputs to pass various parameters through to ci.nix to tell it what to build. Would it be possible to allow passing --override-input inputName url-like:flake/path through to the correct underlying nix command(s)? Right now it looks like I'll have to create a temporary flake that just references the real flake and sets up the overrides.

[Mic92]

This is an alternative: #80
But having an --overide-input option in nix-fast-build is also something I would accept as a pull request.