restricts staging-test-with-rebase to release branches and nightlies

[cfm]

Status

Work in progress

Description of Changes

Towards #5336, restrict staging-test-with-rebase to run only:

1. on pushes to release/* branches; and
2. as part of the nightly workflow.

Testing

To test the restriction to release/* branches:

1. Confirm that staging-test-with-rebase has not run on this branch.
2. Test staging-test-with-rebase's filters.branches.only regex against a known release/ branch.

I'm not aware of a way to test the inclusion of staging-test-with-rebase as a job in the nightly workflow until this PR is merged into develop, but this change is trivial.

Deployment

Development-only; no deployment considerations.

Checklist

If you made non-trivial code changes:

• I have opened a PR in the docs repo for these changes, or will do so later
• I would appreciate help with the documentation
• These changes do not require documentation

Per #5336, these CI changes imply process changes that will need to be determined and documented.

[cfm]

s/Closes/Towards at @conorsch's request. :-)

[zenmonkeykstop]

Approving, thanks for this!