main/postmarketos-ui-os-installer: don't run os-installer and pmbootstrap as root

I'm not sure what the original motivation for doing this was, I would guess just to get a minimal viable product working as soon as possible. But based on my current testing, with these changes along with an earlier commit that disabled the password requirement for privilege escalation with doas, it is unnecessary and also solves at least one bug.

Closes #3848 (closed)