The single repo CI workflow needs to better handle `dbe`

[jcfreeman2]

I'm filing this Issue specifically because of what I observed this morning, which is where dbe was getting cloned as part of the single repo CI workflow actuated by the opening of a PR since it had a johnfreeman/prep_release_merged branch, but since spack load dbe is only run if the repo in which the PR is opened is itself dbe, failures were occuring since spack load dbe is needed to bring in Qt, etc.

See here for an example of what I'm talking about: https://github.com/DUNE-DAQ/fdreadoutlibs/actions/runs/20238328752

Since it's totally normal to envision changes to dbe occurring again during the release period, we'll want to make the workflow robust against this scenario.