generator golden services have same endpoint environment variable

[scotthart]

Both the GoldenThingAdmin and GoldenKitchenSink services use the same env var GOLDEN_KITCHEN_SINK_ENDPOINT. The env var should match the service name. My first thought was this was an issue because both services are defined in the same .proto file. However, Bigquery storage also has two services defined in the same .proto file, but does not suffer from this problem.