CI: add schedule trigger to run once a day

This day it will not catch us by surprise when suddenly
CI fails when the reason is not really related to the
commit being pushed, but by some change done on the GitHub
side.

.github/workflows/CI.yml

@@ -1,6 +1,13 @@
 name: CI
 
-on: [push, pull_request, workflow_dispatch]
+on:
+  push:
+  pull_request:
+  workflow_dispatch:
+
+  # to execute once a day (more info see https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#schedule )
+  schedule:
+    - cron: "0 0 * * *"
 
 # FIXME: figure out why we need to clean after make if we
 # want 'make strict' target to really happen without