ART: Export ASAN_OPTIONS in build testrunner

This script may be run in an environment that hasn't been setup.
If it runs a sanitized setup, it will trigger leak detection in
ninja. Export ASAN_OPTIONS as envsetup.sh does.

Bug: 31098551
Bug: 37751350
Test: art/test/testrunner/run_build_test_target.py -j50 art-asan
Change-Id: I3035d3f5d3df36ad8e1ad79d81ce15a970b6c604