ec2: homedir bug fixed in 24.1.4

This bug fix is due to be released in Quarterly:

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=275896

While here, overwrite the `doas` stanza, which needs to correspond to
the user that we are creating.

MFC after: 5 days
Sponsored by: The FreeBSD Foundation
Pull Request: https://github.com/freebsd/freebsd-src/pull/1163

EC2: Add experimental cloud-init images

Known issues:

1. The ec2-user user is created with a homedir of /usr/home/ec2-user
instead of /home/ec2-user; this appears to be a bug in cloud-init's
FreeBSD support.

2. Cloud-init configures IPv4 networking but not IPv6 networking.

releng/14.0 candidate.

Discussed with: gjb
Reviewed by: imp
MFC after: 5 days
Relnotes: yes
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D41793