Fix deferrable RedshiftClusterSensor

[ferruzzi]

We recently started running the system tests in deferrable mode and noticed the Redshift one fails in that case with a 'coroutine' object is not subscriptable exception. Moving ["Clusters"] to the following line allows the result to parse into a subscriptable object and the test works now.

TLDR:

this fails:

async with self.async_conn as client:
    response = await client.describe_clusters(ClusterIdentifier=cluster_identifier)["Clusters]
    return response

but this works:

async with self.async_conn as client:
    response = await client.describe_clusters(ClusterIdentifier=cluster_identifier)
    return response["Clusters"]

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