Add workflow_dispatch event to GitHub workflows

Decided it'd be nice to manually trigger a workflow, such as when
https://github.com/actions/go-versions/blob/main/versions-manifest.json
receives the latest Go version.

See:
- https://docs.github.com/actions/using-workflows/manually-running-a-workflow
- https://docs.github.com/actions/using-workflows/events-that-trigger-workflows#workflow_dispatch

.github/workflows/run-tests.yaml

@@ -1,6 +1,6 @@
 name: Run Tests
 
-on: [push, pull_request]
+on: [push, pull_request, workflow_dispatch]
 
 jobs:
   ci: