Add workflow_dispatch trigger to the CI build

This is needed to manually trigger the workflow
without having to push a commit to the main branch.

The typical use case is when an upstream dependency
causes a build failure in the latest snapshot and
we want to test the fix by triggering a new build on
github.

.github/workflows/continuous-integration.yml

@@ -1,6 +1,7 @@
 name: CI/CD build
 
 on:
+  workflow_dispatch:
   push:
     branches: [ "main" ]