Fix installation script for A/B system-as-root devices

[dllud]

A/B system-as-root devices have the actual system partition mounted under /system/system when in the recovery.

This should fix both #11 and #12. Though, before closing those issues, we need input from @oscaropenness and @Br31zh to know if they are using A/B system-as-root devices.

Taken from https://gitlab.com/fdroid/privileged-extension/commit/decd7f35ef64e8ec696d2a9feae38fd802479705

[dllud]

My pleasure. Please consider issuing a new release once the affected people at #11 and #12 reply. Or release either way in a couple of weeks in case there's no reply there.

[Roboe]

(For future reference: I did it)