If we actuate a single repo CI workflow from a feature branch, correlated branches should be taken into account

Here's the real-life use case which is prompting this Issue.

• A very simple PR, https://github.com/DUNE-DAQ/trigger/pull/437, failed its automatically-generated-from-a-PR-update single repo CI workflow because there was a legitimate problem with a header in hdf5libs on a branch of the same name
• After fixing the problem with the hdf5libs header, I tried to manually actuate the workflow on the feature branch in the PR
• However, this workflow also failed. This time, the reason is that apparently, even though I actuated the workflow on the feature branch for the PR, the "correlated" repos - including hdf5libs - weren't cloned into the work area, so the fix I'd made in hdf5libs wasn't "seen".

The fact that this is the first time this has come up after a year of handling correlated changes across repos suggests it's not a major problem / urgent, but it would still be nice to have.

Start with the single-repo CI workflow template in this repository and trace how manually actuated feature-branch runs clone correlated repositories. Use the linked trigger PR and the hdf5libs example to understand the missing branch behavior. Done means a workflow run from a feature branch uses the corresponding branches of correlated repositories, including hdf5libs.