ci: push profiles to binary cache

.github/workflows/build.yml

@@ -43,6 +43,12 @@ jobs:
           nix_path: nixpkgs=channel:nixos-unstable
           extra_nix_config: |
             keep-going = true
+      - name: Prepare push to binary cache
+        uses: cachix/cachix-action@v10
+        if: ${{ github.ref == 'refs/heads/master' }}
+        with:
+          name: foo-dogsquared
+          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
       - name: Build Nix profiles
         run: |
           for nixosHost in ni graphical-installer bootstrap; do