HPCC-12094 NamedMutex needs to use correct lock directory

NamedMutex was hard coded to use /var/lock which cannot be
guaranteed to be written to on all distributions.
Instead use the lock directory as specified by the environment.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #6375 from jakesmith/hpcc-12094b

HPCC-12094 NamedMutex needs to use correct lock directory