release: Prepare GCE cloudware images for building as non-root

• Avoid mounting devfs just to list packages and fetch distfiles.
• Add metalog entries.

Reviewed by: cperciva, emaste
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D52455

(cherry picked from commit f1995d6fc13fa4a36f83cea3b90f27bce2aca4d9)