Add check flake action

.gitea/workflows/check-flake.yaml

@@ -0,0 +1,16 @@
+name: Check Flake
+on: [push]
+jobs:
+  check-flake:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Install Nix
+        uses: https://github.com/cachix/install-nix-action@v20
+
+      - name: Checkout the repository
+        uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+
+      - name: Check Flake
+        run: nix flake check