cloud_to_prod ruby interop tests failing

[markdroth]

https://source.cloud.google.com/results/invocations/dd743024-8c20-4459-ba7a-1ec9b8b613f2/targets/github%2Fgrpc%2Finterop_test/tests;group=tests;test=cloud_to_prod_auth:ruby:default:compute_engine_creds:tls;row=1

There are a number of tests failing, but here's one example:

/usr/local/rvm/gems/ruby-2.5.3/gems/google-protobuf-3.10.0-x86_64-linux/lib/google/protobuf.rb:61:in `infer_package': undefined method `size' for nil:NilClass (NoMethodError)
	from /usr/local/rvm/gems/ruby-2.5.3/gems/google-protobuf-3.10.0-x86_64-linux/lib/google/protobuf.rb:109:in `fixup_descriptor'
	from /var/local/git/grpc/src/ruby/pb/src/proto/grpc/testing/test_pb.rb:9:in `add_file'
	from /var/local/git/grpc/src/ruby/pb/src/proto/grpc/testing/test_pb.rb:9:in `block in <top (required)>'
	from /var/local/git/grpc/src/ruby/pb/src/proto/grpc/testing/test_pb.rb:8:in `instance_eval'
	from /var/local/git/grpc/src/ruby/pb/src/proto/grpc/testing/test_pb.rb:8:in `build'
	from /var/local/git/grpc/src/ruby/pb/src/proto/grpc/testing/test_pb.rb:8:in `<top (required)>'
	from /usr/local/rvm/rubies/ruby-2.5.3/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require'
	from /usr/local/rvm/rubies/ruby-2.5.3/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require'
	from /var/local/git/grpc/src/ruby/pb/src/proto/grpc/testing/test_services_pb.rb:23:in `<top (required)>'
	from src/ruby/pb/test/client.rb:42:in `require_relative'
	from src/ruby/pb/test/client.rb:42:in `<main>'

[jtattermusch]

Likely the same problem as #20471