fix installing with mounted /system partition

Moved from https://gitlab.com/postmarketOS/pmaports/-/merge_requests/558, fixes https://gitlab.com/postmarketOS/pmaports/-/issues/418.

Since we're already planning a new release, and this will require modifying the upstream APKBUILD, I thought I'd try to finish this merge request. It's currently set to WIP as I need to verify it actually works.

For those wondering how I managed to keep the commit information: I manually applied the changes from the older commit to the new tree, skipping anything pmaports-specific, commited it, exported as a patch with git format-patch -1, modified it to have the date and author of the original commit, reset my branch and applied the patch with git am.