Retrieve last global tag in IOVDbSvc flags

While running on files produced using different global tags for different steps (like with the q443 test) configFlags.IOVDb.GlobalTag stores the set of all global tags used subsequently. While running subsequently on those, for instance with

runIDPVM.py --filesInput myAOD.pool.root

the following error is triggered when the IOVDbSvc is configured

  File "/cvmfs/atlas-nightlies.cern.ch/repo/sw/master_Athena_x86_64-centos7-gcc11-opt/2022-01-24T2101/Athena/22.0.52/InstallArea/x86_64-centos7-gcc11-opt/python/IOVDbSvc/IOVDbSvcConfig.py", line 48, in IOVDbSvcCfg
    iovDbSvc.GlobalTag=configFlags.IOVDb.GlobalTag
  File "/cvmfs/atlas-nightlies.cern.ch/repo/sw/master_Athena_x86_64-centos7-gcc11-opt/2022-01-24T2101/AthenaExternals/22.0.52/InstallArea/x86_64-centos7-gcc11-opt/python/GaudiConfig2/_configurables.py", line 61, in __set__
    instance._properties[self.name] = self.semantics.store(value)
  File "/cvmfs/atlas-nightlies.cern.ch/repo/sw/master_Athena_x86_64-centos7-gcc11-opt/2022-01-24T2101/AthenaExternals/22.0.52/InstallArea/x86_64-centos7-gcc11-opt/python/GaudiConfig2/semantics.py", line 134, in store
    raise ValueError("cannot set property {} to {!r}".format(self.name, value))
ValueError: cannot set property GlobalTag to ['OFLCOND-MC16-SDR-14', 'OFLCOND-MC16-SDR-RUN2-09']

This MR updates the config to only pass the last global tag in the configuration flags, which fixes the issue.

FYI @tadej @fwinkl @ssnyder