fix(installer): fix `sudo` check for users with password or without privileges

The previous check only worked if the user could run `sudo` without typing the
password, which is almost none (I checked in Google Cloud Shell so I failed to
notice this).

This new check works whether the user has no sudo privileges, or if it has,
whether they have to type in the password or not.

It should really be easier to check if the user doesn't have privilege without
having to make them type the password.