Download .asc files for rustup channels

[k3d3]

There are .asc files that exist alongside many of the rustup files. For this issue, these files don't need to be verified, just downloaded.

To add to this issue - it doesn't appear that panamax pulls in `.asc` files for rustup channels.

_Originally posted by @DrChat in https://github.com/panamax-rs/panamax/issues/7#issuecomment-915617132_