[actions] Add flake check

.gitea/workflows/flake-check.yml

@@ -0,0 +1,15 @@
+name: "Check flake syntax"
+on:
+  pull_request:
+  push:
+jobs:
+  flake-check:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - name: Install dependencies for Nix setup action
+      run: |
+        apt update -y
+        apt install sudo -y        
+    - uses: cachix/install-nix-action@v27
+    - run: nix flake check --no-update-lock-file --accept-flake-config .