Don't upload to S3 on local cache hit

[franklin-ross]

When using S3 with save-cache: true and the local cache hits, we seem to then upload the local cache to S3 in the post-command step. That doesn't seem necessary as a local cache hit implies the same entry already exists in S3 and was previously downloaded.

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For comparison, here's what happens when the local cache misses and we download from S3, no 10 second upload:

[calvinbui]

i'm taking a look

[franklin-ross]

🎉