Set the GRPC_DEFAULT_SSL_ROOTS_FILE_PATH env var in initializer

What does this MR do and why?

The root certificate that the grpc ruby gem uses for secured communications expired. Setting the GRPC_DEFAULT_SSL_ROOTS_FILE_PATH environment variable will set it for all gRPC connections. Here we're setting it to the system's default certificate which should be more up-to-date, or at least easier to update.

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Related to #342346 (closed)