Support manual dispatch

Author: philwebb

.github/workflows/ci.yml

@@ -16,6 +16,7 @@ jobs:
   activate:
     if: >-
       github.event_name == 'push' ||
+      github.event_name == 'workflow_dispatch' ||
       (github.event_name == 'schedule' && github.repository_owner == 'spring-io') ||
       (github.event_name == 'pull_request' && !startsWith(github.head_ref, 'docs/'))
     runs-on: ubuntu-latest