ci: re-run Go checks for every push on a pull request

If a pull request has changed any Go file, even if the new push doesn't change any,
we should re-run Go checks to make GitHub Checks look consistent.

.github/workflows/go.yml

@@ -3,6 +3,9 @@ on:
   push:
     paths:
       - '**.go'
+  pull_request:
+    paths:
+      - '**.go'
 env:
   GOPROXY: "https://proxy.golang.org"