Add buildifier to sanity_tests.yaml

tools/run_tests/sanity/sanity_tests.yaml

@@ -1,6 +1,7 @@
 # a set of tests that are run in parallel for sanity tests
 - script: tools/run_tests/sanity/check_bad_dependencies.sh
 - script: tools/run_tests/sanity/check_bazel_workspace.py
+- script: tools/run_tests/sanity/check_buildifier.sh
 - script: tools/run_tests/sanity/check_cache_mk.sh
 - script: tools/run_tests/sanity/check_deprecated_grpc++.py
 - script: tools/run_tests/sanity/check_owners.sh