Use cachix to cache builds

CI is getting too slow, rebuilding too many derivations, especially now
that we depend on a patched treefmt (ironically the patch is there to
speed it up)

Author: infinisil

.github/workflows/main.yml

@@ -35,6 +35,11 @@ jobs:
 
       - uses: cachix/install-nix-action@v26
 
+      - uses: cachix/cachix-action@v14
+        with:
+          name: nixos-nixfmt
+          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+
       - name: checks
         run: nix-build -A ci
 
@@ -75,6 +80,11 @@ jobs:
 
       - uses: cachix/install-nix-action@v26
 
+      - uses: cachix/cachix-action@v14
+        with:
+          name: nixos-nixfmt
+          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+
       - run: |
           ./scripts/sync-pr.sh \
             https://github.com/${{ github.repository }} \