Replace async-tar dependency

[rylev]

We're currently relying on a fork of async-tar - it seems because of previous Windows build failures. It would be best to replace this dependency with a more maintained version. Ideally we would replace async-tar entirely as it is currently the only dependency bringing in async-std which itself has ~5 dependencies we don't otherwise use. It might make sense to switch to the sync tar crate and use tokio::sync::spawn_blocking where appropriate.

[itowlson]

We are now back on official async-tar but the broader issue about async-std remains.