HPCC-12010 Fix deadlock in onActivity cache building

If building, whilst a queue request is processing, e.g.
onSetJobPriority, the two threads involved can deadlock each
other.
Regression introduced by HPCC-11637

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
I'd argue it's consistent (or will be) with proceeding the object that it's trying to scope, namely the IJobQueue