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
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation

usr.sbin/freebsd-update/freebsd-update.sh

@@ -3492,6 +3492,9 @@ fi
 # Set LC_ALL in order to avoid problems with character ranges like [A-Z].
 export LC_ALL=C
 
+# Clear environment variables that may affect operation of tools that we use.
+unset GREP_OPTIONS
+
 get_params $@
 for COMMAND in ${COMMANDS}; do
 	cmd_${COMMAND}