Avoid running the workflow twice on PR pushes

When someone force-pushes to a PR or adds a new commit, the workflow would run
_twice_, as both `push` and `pull_request` events would trigger.

The default for `types` is `[opened, reopened, synchronize]`; the latter is the
type that overlaps with `push`.

.github/workflows/ci.yml

@@ -10,6 +10,7 @@ on:
     branches:
       - master
       - ?.?*
+    types: [opened, reopened]
   schedule:
     - cron:  '0 6 * * *'  # Daily 6AM UTC build