Enable nightly CI jobs (#263)

For now I've only added the standalone builds and tests.

This should give us more insight on flaky tests. Also, this runs the URL
check so we find broken links earlier.

Author: urfeex

.github/workflows/ci.yml

@@ -5,6 +5,8 @@ on:
   push:
     branches:
       - master
+  schedule:
+    - cron: '38 2 * * *'
 
 jobs:
   build: