Add --build-variant to create_reference_dumps.py

create_reference_dumps.py supports --build-variant which defaults to
userdebug. When the script builds libraries, it sets
TARGET_BUILD_VARIANT instead of inheriting from current environment.

Bug: 121017205
Test: ./create_reference_dumps.py --build-variant eng --llndk -libs libbinder_ndk
Change-Id: Ibe5458c2bb62ebb44755e4bb90566d39a0bd52f2
2 files changed