modprobe: shouldn't load kernel modules from /system

Kernel modules are not permitted to be on /system partition.
That was one of Treble requirements in O:
https://source.android.com/devices/architecture/kernel/modular-kernels#file-locations

Bug: 74069409
Test: pixel/nexus devices don't have LKMs in /system, so this change
shoudl be harmless.
Test: walleye boots without issues from modprobe.
Change-Id: I8b3aeb55aacb3c99e0486224161d09a64bb52cd1