add check runner

.gitea/workflows/checks.yaml

@@ -0,0 +1,12 @@
+name: checks
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+jobs:
+  checks-impure:
+    runs-on: nix
+    steps:
+      - uses: actions/checkout@v3
+      - run: nix flake check