Add Nix flake

[mattwynne]

This adds a flake.nix that allows users to install gitshorty using Nix flakes. Users can now:

• Install directly: nix profile install github:carpeliam/gitshorty
• Use in flake-based projects as an input
• Run without installing: nix run github:carpeliam/gitshorty

The flake builds the project using buildGoModule and renames the binary from gitshorty to sc to match the project's conventions.

Also updates README.md with installation instructions for Nix users.

[carpeliam]

Thanks @mattwynne! I'm not too familiar with the nix ecosystem, so I have a few questions. Is the SHA attached to the HEAD commit, or the commit at tag v0.0.4? In other words, if I push a new commit that's not ready for release, what does nix do? And, do you recommend any processes for keeping the flake up to date as new releases come out? I don't think having GHA in place should be a precursor to getting this merged, but I'd like to better understand the complexity.

[carpeliam]

After looking around a bit, it looks like the vendor hash follows my dependencies, but I just merged a dependabot update of some dependencies. Does that mean this is already out of date? From a cursory glance, it looks like the best way to update the flake is to use nix itself; if that's the case, I'll have to look into using GHA to keep the flake updated, as I'd like to avoid installing nix on my personal machine.