ota zip broken on twrp 3.5.2

2 years ago, I submitted a patch to fix ota zip installation on system-as-root devices.

I have today updated my device (OP6) to LOS18.1 and twrp 3.5.2. (Previously it was LOS17.1 and twrp 3.3.1)

Turns out, the system-as-root mount behavior has been changed yet again, which breaks the OTA zip (again).

$ getprop ro.build.system_root_image
true

$ mount
/dev/block/sda13 on /system_root type ext4 (rw,seclabel,relatime)
/dev/block/sda13 on /system type ext4 (rw,seclabel,relatime)

The system-as-root partition is now mounted to /system_root, and what was previously /system/system is now only /system.

That results in this part of the update-binary not working correctly anymore, which breaks the whole installation.