release: Complete NO_ROOTification of Vagrant builds

• Don't clean pkg files, they won't be recorded in the manifest anyway.
• Use pw's new metalog mode to create the vagrant user. Note that we do not need to manually create the home directory, pw will do it, so just remove that.
• Write metalog entries for the vagrant user's ssh key dir and authorized keys file

Note, this depends on an updated pw being installed on the host.

Reviewed by: emaste
MFC after: 1 day
Sponsored by: Klara, Inc.
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52631

(cherry picked from commit 02dfca2458081587d8224c5ebf8bde99fa360f94)