freebsd-update: avoid effects of user's environment on grep

A user had GREP_OPTIONS containing --color=always, which broke grep use
in freebsd-update. Unset this environment variable.

PR: 255990
Reviewed by: kevans
Approved by: re (cperciva)
Sponsored by: The FreeBSD Foundation

(cherry picked from commit e093c61b2d69e9cca54a2332096c0e36f97bffc0)
(cherry picked from commit 24fe8a518324e291b9ea3fe4e60f2e0fb4f857c5)