freebsd-update: use grep -E instead of egrep

GNU egrep emits a warning that it is obsolescent and suggests grep -E
instead. Switch to grep -E in case we end up invoking GNU (e)grep (and
for consistency with other invocations in this file).

Reported by: Steffen Nurpmeso
Approved by: re (cperciva)
Sponsored by: The FreeBSD Foundation

(cherry picked from commit e27ded83c76a609687a3d9e82b80fe7e1b782bf6)
(cherry picked from commit b562307b70346030f59fe6a05d125814c74da47b)