Add basic action for building and testing

.gitea/build.yaml

@@ -0,0 +1,15 @@
+name: Build library & run tests
+on: [push]
+
+jobs:
+  build:
+    runs-on: nix
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+      - name: Build & test
+        run: nix build
+      - name: Upload result
+        uses: actions/upload-artifact@v3
+        with:
+          name: result