sysutils/amazon-ssm-agent: Use pw(8)'s new metalog feature

This allows the ssm-agent user to be created when installing as an
unprivileged user. That is, if the package is installed with pkg(8)'s
METALOG option set, the home directory for ssm-agent will be added to
the metalog.

PR: 290080
Approved by: cperciva (maintainer)
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.