setup gitea workflow

.gitea/workflows/build.yaml

@@ -0,0 +1,15 @@
+name: Nix Build
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+jobs:
+  build:
+    runs-on: nixos-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Build
+        run: nix build