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, expedited MFC)
Sponsored by: The FreeBSD Foundation

(cherry picked from commit e093c61b2d69e9cca54a2332096c0e36f97bffc0)