fix #11693: prevent rename of user's current group

This attempts to ameliorate https://trac.openmicroscopy.org.uk/ome/ticket/11693 but do note https://trac.openmicroscopy.org.uk/ome/ticket/11693#comment:8 (responding to the previous) which may mean that this PR is taking the wrong approach. Still, hopefully it at least stimulates useful discussion. Cc: @dpwrussell.