STT build test builds trunk_staging release by default

Add `TARGET_RELEASE=trunk_staging` for the base build command to
build the trunk_staging targets for the STT build tests.

Bug: 293964685
Test: combined_build_test.sh -t <target> -v <variant>
Change-Id: I1bc449ad6790dc56860983ae80c4beb20c0621aa
1 file changed