Promotion shouldn't use curl to download manifests

[aidanhs]

This may cause it to download older manifests that we've hacked around with by hand and forgotten to flush the cache of.

Instead it should use aws s3 cp --cache-control none.

cc @Mark-Simulacrum

[Mark-Simulacrum]

Note that --cache-control none is entirely arbitrary: I have no idea if none is the right thing to pass there.

[alexcrichton]

Ah using aws s3 cp should be fine, that goes straight to S3 and avoids all caches