Add workflow_dispatch trigger to GitHub Actions CI (#9)

This allows users to trigger CI runs at any time from the GitHub Actions
UI. We don't publish anything as part of the colcon/ci run, so it's safe
to do and sometimes useful to investigate changes in our dependencies.

.github/workflows/ci.yaml

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