This adds a nix workflow that does the following:
* Checks to see if the Zig cache fixed derivation needs its hash
updated. It does this by downloading a new cache, calculating the
hash, and comparing it against what has already been set. If the hash
is different, the workflow fails.
* Runs a Nix build if the hash is OK, testing the build of
packages.ghostty (using "nix build .")
The cache workflow we use in the build job comes from:
https://github.com/DeterminateSystems/magic-nix-cache-action/Fixes#937.
