Don't use the default boot image location with non-default bootclasspath am: 6023b3a53a

Original change: https://android-review.googlesource.com/c/platform/art/+/2463234

Change-Id: I5868a81a6ba9d3d423e87d9105b410d60afee91e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>