Because Zig does not fetch recursive dependencies when you run `zig build
--fetch` (see https://github.com/ziglang/zig/issues/20976) we need to do some
extra work to fetch everything that we actually need to build without Internet
access (such as when building a Nix package).
An example of this happening:
```
error: builder for '/nix/store/cx8qcwrhjmjxik2547fw99v5j6np5san-ghostty-0.1.0.drv' failed with exit code 1;
la/build/tmp.xgHOheUF7V/p/12208cfdda4d5fdbc81b0c44b82e4d6dba2d4a86bff644a153e026fdfc80f8469133/build.zig.zon:7:20: error: unable to discover remote git server capabilities: TemporaryNameServerFailure
> .url = "git+https://github.com/zigimg/zigimg#3a667bdb3d7f0955a5a51c8468eac83210c1439e",
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> /build/tmp.xgHOheUF7V/p/12208cfdda4d5fdbc81b0c44b82e4d6dba2d4a86bff644a153e026fdfc80f8469133/build.zig.zon:16:20: error: unable to discover remote git server capabilities: TemporaryNameServerFailure
> .url = "git+https://github.com/mitchellh/libxev#f6a672a78436d8efee1aa847a43a900ad773618b",
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
For full logs, run 'nix log /nix/store/cx8qcwrhjmjxik2547fw99v5j6np5san-ghostty-0.1.0.drv'.
```
To update this script, add any failing URLs with a line like this:
```
zig fetch <url>
```
Periodically old URLs may need to be cleaned out.
Hopefully when the Zig issue is fixed this script can be eliminated in favor
of a plain `zig build --fetch`.
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.
