pkg-stage: Correct /packages permissions

Include /packages in the METALOG used to create dvd1.iso. Previously
we used an expression ^./packages/ (with a trailing /) which did not
match /packages itself, and then with no METALOG entry /packages on
dvd1.iso ended up with mode d---------.

PR: 290222
Reviewed by: cperciva
MFC after: 1 minute
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 2db11dda840c4889aeb719761cdafa62730083a2)