Check flake instead of dry building

.gitea/workflows/check.yml

@@ -0,0 +1,15 @@
+name: Check flake
+on:
+  push:
+    branches: ['master']
+jobs:
+  build-amd64-linux:
+    runs-on: nix
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          ref: master
+      - name: Check flake
+        run: |
+          echo "Checking flake at ${{ gitea.ref }}"
+          nix flake check