common/auth: add --force to pam-auth-update exec

This is mostly a problem when bringing up a new desktop; we need to
enable the "mkhomedir" PAM profile, and if we run `pam-auth-update`
manually, we get a nice menu prompt to enable it. But just running
`pam-auth-update --package` won't actually enable it, since it thinks
we've overriden it (or something). Adding --force fixes that.