test(spanner): deflake samples against production (#5702)
test(spanner): deflake samples against production (#5702)

Several builds run against production and run relatively slow
operations, such as creating instances. With this change different
builds will pick a different location, and hopefully distribute the load
more fairly.

googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp