Fix broken enable_lz4diff flag

When enable_lz4diff=true, we need to inject lz4.so from target_files to
delta_generator, this is done by setting LD_PRELOAD.
We used to rely on brillo_update_payload for setting LD_PRELOAD,
since brillo_update_payload is removed, in aosp/2726801, we
need to do this in python

Test: th
Bug: 382199131
Change-Id: I3696363408cf990d816011c0b034c3fc94f14943