[ci skip] CI: add workflow_dispatch trigger

This should make it possible to manually re-trigger stale PR workflows,
and might occasionally be useful to do a manual build against latest
PHP versions (and as such is a poor man's substitute for the nightly
builds which have been disabled recently, because apparently nobody had
a look at the results anyway).

.github/workflows/ci.yml

@@ -1,5 +1,5 @@
 name: Build and Test
-on: [push, pull_request]
+on: [push, pull_request, workflow_dispatch]
 jobs:
   ubuntu:
     strategy: