release: Prepare EC2 cloudware images to be buildable as non-root

• Avoid chrooting to uninstall pkg, as it will not be installed in the first place when in non-root mode.
• Add metalog entries.

Reviewed by: cperciva
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D52452