lakitu: Use usermod to add users to groups in google-guest-agent

We found out a bug in gpasswd that may cause modifications of
/etc/group from other programs to be lost. This change configures
google-guest-agent to use usermod instead of gpasswd to add users
to groups.

BUG=b/176502851
TEST=presubmit
RELEASE_NOTE=Configured google-guest-agent to use usermod instead
of gpasswd to add users to groups.

Change-Id: I33683601dd92e63f5c7f09f736ab4c9a7259e420
Reviewed-on: https://cos-review.googlesource.com/c/cos/overlays/board-overlays/+/15130
Reviewed-by: Dexter Rivera <riverade@google.com>
Reviewed-by: Vaibhav Rustagi <vaibhavrustagi@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
2 files changed