GH Actions: allow for manually triggering a workflow

Triggering a workflow for a branch manually is not supported by default in GH Actions, but has to be explicitly allowed.

This is useful if, for instance, an external action script or composer dependency has broken.
Once a fix is available, failing builds for open PRs can be retriggered manually instead of having to be re-pushed to retrigger the workflow.

Ref: https://github.blog/changelog/2020-07-06-github-actions-manual-triggers-with-workflow_dispatch/

.github/workflows/php-qa.yml

@@ -1,5 +1,8 @@
 name: PHP Quality Assurance
-on: [push]
+on:
+  push:
+  # Allow manually triggering the workflow.
+  workflow_dispatch:
 
 # Cancels all previous workflow runs for the same branch that have not yet completed.
 concurrency: