This PR removes most of the logic of nightly_matrix.php by converting
nightly.yml to a workflow_call, and invoking it multiple times for each branch
in a new root.yml workflow. The naming of the files is intentionally kept to
make the diff readable. They may still be renamed afterward.
Closes GH-16642
These files are usually only used on master, as the cron trigger runs on master.
However, workflow_dispatch does use the file from the triggered branch.
Furthermore, we're looking into migrating to a workflow_call trigger, which will
also run on the target branch. Backport these files now to make those changes
easier.
