build_image: Do not use base_fs file for system_other.

system_other is not OTA'd (outside of factory OTA), and its block map differs greatly from
system.img, so do not attempt to re-use it.

Bug: 139201772
Test: make with base_fs file for system
Change-Id: I1ed2091da381ebdb93522a081e5d2f7c18c9d069