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

treble/combined_build_test.sh

@@ -78,7 +78,7 @@ if [[ -n "${dist_dir}" ]]; then
     out_dir="${dist_dir}/logs"
 fi
 
-base_command="build/soong/soong_ui.bash --make-mode"
+base_command="build/soong/soong_ui.bash --make-mode TARGET_RELEASE=trunk_staging"
 if [[ -n "${dist_dir}" ]]; then
     base_command="${base_command} DIST_DIR=${dist_dir} dist"
 fi