devel/qt6-base: repair timezone-loading

This is basically the same patch as in devel/qt5-core, but
written anew and a little more compact. On FreeBSD, prefer
reading from /var/db/zoneinfo which contains the name of the
system timezone. (And /etc/localtime is a complete tzinfo file,
not suitable for finding the name in a way suitable for this
bit of code).

Reported-by: Martin Birgmeier
PR: 285244