[ci] Run the CI daily on main branch

This has two benefits: First, we ensure that the main branch works even
if the runners/dependencies update. Second, GitHub's caches are scoped.
They only work among commits of the same branch, except from the default
branch. (https://github.com/actions/cache#cache-scopes) Therefore, if we
ensure that the main branch is always cached, we should always hit the
caches and have quick turnaround times on all our branches and PRs.

.github/workflows/ci.yml

@@ -5,6 +5,11 @@
 name: ci
 on: [push, pull_request]
 
+on:
+  schedule:
+    # Run the CI on the main branch every morning at 01:19 UTC
+    - cron:  '19 1 * * *'
+
 jobs:
 
   ################