ci: Allow triggering Go workflow manually

External contributors don't do development on the main or develop
branch, but may still need to see the CI results. This lets them
do `gh workflow run Go --repo=<user>/copy` on their own branches.

.github/workflows/go.yml

@@ -5,6 +5,7 @@ on:
     branches: [ main, develop ]
   pull_request:
     branches: [ main, develop ]
+  workflow_dispatch:
 
 jobs: