freebsd-update: Skip /etc/ssl/cert.pem

We already run `certctl rehash` at the end, there is no point in asking
users upgrading from 15.0 to 15.1 to manually merge the trust store.

MFC after:	3 days
Reviewed by:	cperciva
Differential Revision:	https://reviews.freebsd.org/D57028

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

@@ -2581,7 +2581,7 @@ upgrade_merge () {
 
 			# Some files need special treatment.
 			case ${F} in
-			/etc/spwd.db | /etc/pwd.db | /etc/login.conf.db)
+			/etc/spwd.db | /etc/pwd.db | /etc/login.conf.db | /etc/ssl/cert.pem)
 				# Don't merge these -- we're rebuild them
 				# after updates are installed.
 				cp merge/old/${F} merge/new/${F}