Fix test failures for sharded cluster without replica set

This build failure for PHPC-1369 is likely related to retryable writes being enabled by default but the sharded cluster not using a replica sets. Many tests failed with the following unexpected error:

Transaction numbers are only allowed on a replica set member or mongos

Since libmongoc only communicates with mongos, it doesn't detect that the cluster doesn't actually support transaction numbers. We can either remove the SHARDED_CLUSTER environment or manually work around the issue by explicitly disabling retryable writes for that environment.