Move the C++ test build to CI build job instead of test job (#5442)

Author: JackCaoG

.circleci/build.sh

@@ -48,6 +48,7 @@ sccache --show-stats
 source $XLA_DIR/xla_env
 export GCLOUD_SERVICE_KEY_FILE="$XLA_DIR/default_credentials.json"
 export SILO_NAME='cache-silo-ci-gcc-11'  # cache bucket for CI
+export BUILD_CPP_TESTS='1'
 build_torch_xla $XLA_DIR
 
 popd