Limit Bazel memory usage in CI.

This aims to reduce/eliminate the CircleCI flakes described in grpc-ecosystem#968.
It's based on the approach in
https://github.com/angular/angular-cli/blob/master/.circleci/bazel.rc

.circleci/config.yml

@@ -69,6 +69,8 @@ jobs:
             startup --output_base $HOME/.cache/_grpc_gateway_bazel
             build --test_output errors
             build --features race
+            # Workaround https://github.com/bazelbuild/bazel/issues/3645
+            build --local_resources=3072,2.0,1.0
             EOF
       - run:
           name: Check that Bazel BUILD files are up-to-date