ci: add flake validation job before staging VM provisioning

.forgejo/workflows/staging.yml

@@ -5,8 +5,24 @@ on:
     types: [opened, synchronize, closed]
 
 jobs:
+  validate-flake:
+    runs-on: self-hosted
+    steps:
+      - name: Checkout Repository
+        uses: actions/checkout@v4
+
+      - name: Install Nix
+        uses: cachix/install-nix-action@v27
+        with:
+          extra_nix_config: |
+            experimental-features = nix-command flakes
+
+      - name: Validate Flake
+        run: nix flake check
+
   manage-staging:
     runs-on: self-hosted
+    needs: validate-flake
     steps:
       - name: Checkout Repository
         uses: actions/checkout@v4