Stop hardcoding `in-pmbootstrap` to check chroot status in favor of checking if current root is equal to the root of the `init` process (PID 1)

Needs testing before merging.

Idea taken from here: https://unix.stackexchange.com/a/14346

Relevant comment thread: https://gitlab.com/kupfer/packages/pkgbuilds/-/merge_requests/77#note_1870089440

CC: @Newbyte @craftyguy