ci: Skip builds on push on main (#1276)

These builds were already tested on pr merge so why run again?

Reviewed-on: https://gitea.com/gitea/runner/pulls/1276
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>

.gitea/workflows/test.yml

@@ -1,9 +1,7 @@
 name: checks
 on:
-  push:
-    branches:
-      - main
   pull_request:
+  workflow_dispatch:
 
 env:
   # The runner image ships a stale docker.io login; point docker at an empty config so