main/postmarketos-initramfs: fix filesystem detection

Changes the regex of the initramfs to make sure that it is searching for filesystem type instead of any type (e.g. partition scheme type).

Currently while trying to find the filesystem type of a partition the regex of the initramfs can instead get fed the partition scheme type (e.g. gpt), causing it to error with an unsupported filesystem error.

Fixes #3303 (closed)