xla

0986ec50 - Fix nightly whl build issue & introduce new knobs in cloudbuild.yaml (#4427)

Fix nightly whl build issue & introduce new knobs in cloudbuild.yaml (#4427) * Overwrite mkl pakcages during whl build. * Enable _GITHUB_XLA_BRANCH in docker/cloudbuild.yaml * Enable _GITHUB_EXAMPLE_BRANCH in docker/cloudbuild.yaml * Enable _BUILD_CPP_TESTS in docker/cloudbuild.yaml