Improve fetching build variable `TARGET_ARCH` in `buildbot-build.sh`.

Use `build/soong/soong_ui.bash` instead of sourcing
`build/envsetup.sh`, which also plays nicely with the use of
`shopt -s failglob` in script `tools/buildbot-build.sh` (script
`build/envsetup.sh` seems to have introduced new completion features
which are incompatible with `build/envsetup.sh`'s use of
`shopt -s failglob`.)

Test: art/tools/buildbot-build.sh --target
Change-Id: Ifddc05a505f66298fcd476649ab687d4bdd84984
1 file changed