Use magic-nix-cache-action

This should remove the need to manually check if the build output exists
in the cachix cache since it would be restored from the GitHub Actions
cache instead and therefore exists locally already.

.github/workflows/build-flake-outputs.yml

@@ -48,6 +48,7 @@ jobs:
             accept-flake-config = true
             always-allow-substitutes = true
             ${{ startsWith(matrix.os, 'ubuntu') && 'extra-platforms = aarch64-linux' || ' ' }}
+      - uses: DeterminateSystems/magic-nix-cache-action@v3
       - uses: cachix/cachix-action@v14
         with:
           name: matrss
@@ -61,21 +62,7 @@ jobs:
             } 3>&1
           )
           echo "drv=$drv" >> "$GITHUB_ENV"
-      - name: Get output hash
-        run: |
-          hash=$(nix derivation show $drv^* | jq -r ".\"$drv\".outputs.out.path" | cut -d/ -f4 | cut -d- -f1)
-          echo "hash=$hash" >> "$GITHUB_ENV"
-      - name: Check if output is cached
-        run: |
-          is_cached() {
-            curl --fail "https://matrss.cachix.org/$hash.narinfo" > /dev/null 2>&1
-          }
-          if is_cached $hash; then
-            echo "output is cached, nothing to do"
-            echo "skip_build=true" >> "$GITHUB_ENV"
-          fi
       - name: Build output
-        if: ${{ env.skip_build != 'true' }}
         run: nix build -L $drv^*
       - name: Check if there were warnings
         if: ${{ github.ref != 'refs/heads/main' }}