DataQualityDict.populate doesn't respect source keyword properly

The DataQualityDict.populate method doesn't fully respect the source keyword argument, because it isn't passed into the per-flag query() call:

https://github.com/gwpy/gwpy/blob/ddb01aed2bec0b21ef18da488e5da100c963c6e5/gwpy/segments/flag.py#L1442-L1445

This means that the per-flag query always goes to DEFAULT_SEGMENT_SERVER, rather than what the user told it to.

Reported originally for LIGO in: https://git.ligo.org/computing/helpdesk/-/issues/4536