Move Nightly Library installs to GH Actions

This is still an admin cronjob, but is similar enough to nightly install of compilers that it can move to GH actions. This would make it a lot more transparent as to what's going right/wrong.

The question is if it reveals any secrets? and if so, what can we do about it

Start by comparing the existing admin cronjob for nightly library installs with the nightly compiler installs already running in GitHub Actions. Determine whether the job exposes secrets, then define a workflow that makes success and failure transparent without exposing them; no specific files or tests are named in the issue.

Written by the indexing model from the issue text.