Installer script ignores `INSTALL_PARTITION` variable

At pmos_install_functions#L51, the installer ignores the variable INSTALL_PARTITION and selects the system partition as the target in all cases, except when findfs fails and the script falls back to parsing the fstab.

The result of this, is that the pmbootstrap command line option --recovery-install-partition might fail to achieve its effect. (mentioning only to inform pmbootstrap users, the bug is not in pmbootstrap)

See https://gitlab.com/postmarketOS/pmaports/issues/382#note_235163323 .