Use build-flag from build/soong/bin

During lunch, place build/soong/bin at the head of
ANDROID_LUNCH_BUILD_PATHS, so that running lunch fixes the issue for
soong-only builds.

Remove {HOST_OUT}/bin/build-flag in CleanSpec.mk so that any
non-Soong-only build will remove the colliding file, without requiring
lunch.

Bug: b/422743543
Test: manual, TH
Change-Id: Id36fef029a182d915224d086a291701bd1637c3d