ci: update FreeBSD image to 14.3

ci: update FreeBSD image to 14.3

FreeBSD 13.4 is no longer supported, and 13.5 will be the last
release from that series, so jump instead to 14.3 which should
be supported for another 10 months and will be at that point
the oldest supported release with the interim release of 15.

While at it, move some variables to the environment and make
sure to skip a git grep test that assumes glibc regex.

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>