Add daily CI jobs (#499)

* Add daily CI jobs (cf. oscar-system/Oscar.jl#4072)

* Update .github/workflows/oscar.yml

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Co-authored-by: Benjamin Lorenz <benlorenz@users.noreply.github.com>

.github/workflows/runtests.yml

@@ -8,6 +8,9 @@ on:
  pull_request:
  branches:
  - master
+ schedule:
+ # Every day at 3:05 AM UTC
+ - cron: '5 3 * * *'
  workflow_dispatch:
 
 # needed to allow julia-actions/cache to delete old caches that it has created