ci: add Cachix cache (#919)

This will improve CI performance, and may also be used locally,
although this is not officially documented at this time.

I tested the required amount of space by building a `pkgs.linkFarm`
containing all testbeds, then manually pushing this to an empty Cachix
cache.

The total space used was 274 MiB of the 5 GiB free allowance, so
Cachix is good enough to cache at least 20 commits of history,
assuming the worst case scenario of every commit changing
every derivation we build.

Fixes #880

.github/workflows/docs.yml

@@ -25,7 +25,14 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
+
       - uses: DeterminateSystems/nix-installer-action@v16
+
+      - uses: cachix/cachix-action@v15
+        with:
+          name: stylix
+          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+
       - run: nix build .#docs
 
       - uses: actions/upload-pages-artifact@v3