Experiment with better CI caching (#544)

* Experiment with better CI caching

cache `aws-sdk/target`

* Update ci.yaml

try point cargo at `../target`

* setup proper restore-keys

* Delete unused cache, force incremental

* expiriment with pre-tarring

* Fix tar

* the download is actually a folder

* add untar to all tasks

* tar the artifact produced by generating a complete SDK

* Update ci.yaml

experiment with only testing s3

* Only test a subset of services