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