.github: Make workflow run also on PRs, rename push->build

.github/workflows/push.yaml → .github/workflows/build.yaml

@@ -1,6 +1,10 @@
 name: Go
 
-on: [push]
+on:
+  pull_request:
+  - push
+  push:
+    branches: [main]
 
 # This concurrency rule is not really relevant here but is nice for
 # cut-n-pasting to elsewhere