add ci and renovate

.gitea/workflows/nix-flake-check.yaml

@@ -0,0 +1,14 @@
+name: nix flake check
+on:
+  push:
+    branches:
+      - 'renovate/**'
+
+jobs:
+  nix-flake-update:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install Nix
+        uses: cachix/install-nix-action@v27
+      - run: nix flake check --all-systems