TrigValSteering: only use references from eos if readable

Frank Winklmeierrequested to merge
fwinkl/athena:trigvalref into 24.0

When deciding from where to read references (eos/cvmfs) we currently only check if the file exists. But it should also be readable.

Fixes problem seen in the HLTMPPU CI (atlas-tdaq-software/HLTMPPU!44 (merged)).

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