ci: add flake check

.github/workflows/nix-pr-check.yml

@@ -0,0 +1,30 @@
+name: Check nix flake
+
+on:
+  pull_request:
+    branches: [master, main]
+    paths:
+      - "flake.*"
+      - "distro/nix/**"
+jobs:
+  check-flake:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Create GitHub App token
+        id: app_token
+        uses: actions/create-github-app-token@v1
+        with:
+          app-id: ${{ secrets.APP_ID }}
+          private-key: ${{ secrets.APP_PRIVATE_KEY }}
+
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+          token: ${{ steps.app_token.outputs.token }}
+
+      - name: Install Nix
+        uses: cachix/install-nix-action@v31
+
+      - name: Update vendorHash in flake.nix
+        run: nix flake check