fixes #3611 #3511

@spikecurtis explains the problem well:

Ok, we've chased this down. The issue is when the Coder user is named admin, the cloud-init fails because it tries to create user=admin group=admin, but admin is the name of a builtin group.

We could just hardcode the username "coder" in our templates, or possibly hardcode just the group "coder" (rather than the default of creating a group with the same name as the user).

I tested manually on both templates and confirmed the templates now work with an admin username in Coder, where they previously didn't before.