Showcase presubmits failing at HEAD with `Unable to parse.`

[parthea]

See build log here in PR #2508.

nox > python -m grpc_tools.protoc --experimental_allow_proto3_optional --descriptor_set_in=/tmp/tmp6dd2y5qs/showcase.desc --python_gapic_opt=python-gapic-templates=DEFAULT,transport=grpc+rest,service-yaml=/tmp/tmp6dd2y5qs/showcase_v1beta1.yaml,retry-config=/tmp/tmp6dd2y5qs/showcase_grpc_service_config.json --python_gapic_out=/tmp/tmp6dd2y5qs google/showcase/v1beta1/echo.proto google/showcase/v1beta1/identity.proto google/showcase/v1beta1/messaging.proto
/tmp/tmp6dd2y5qs/showcase.desc: Unable to parse.

Potentially related to #2473

[parthea]

I'm not able to reproduce this issue locally and it's no longer occurring in #2508. The problem may have been a transient issue with the test environment. It's difficult to identify the root cause since we don't have a failed state.