set USE_RBE=false in Soong integration tests

RBE is already disabled for this test on CI. Locally, the mock checkout
created for the Soong integration tests does not set up RBE properly, so
we should just disable it explicitly so that the integration tests can
be run locally.

Test: build/soong/tests/run_integration_tests.sh
Change-Id: I38875a02b549a9d8c8368d6287c4516518d20e34

tests/lib.sh

@@ -111,7 +111,7 @@ function setup {
 
 # shellcheck disable=SC2120
 function run_soong {
-  build/soong/soong_ui.bash --make-mode --skip-ninja --skip-config --soong-only --skip-soong-tests "$@"
+  USE_RBE=false build/soong/soong_ui.bash --make-mode --skip-ninja --skip-config --soong-only --skip-soong-tests "$@"
 }
 
 function create_mock_bazel {