CI: add workflow_dispatch & schedule triggers

.github/workflows/CI.yml

@@ -1,6 +1,14 @@
 name: CI
 
-on: [push, pull_request]
+on:
+  push:
+  pull_request:
+  workflow_dispatch:
+
+  # see https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#schedule
+  schedule:
+    # once a day
+    - cron: "0 0 * * *"
 
 jobs: