- Add `update-index` subcommand to generate registry index from `.crate` files

- Change `mirror` to not grab crates.io git index by default, using `update-index` subcommand is recommended

- Ensures the git index correctly reflects the partial crates.io mirror

- Enables anyone to update and bring their own .crate files, and fix/add to the index on their own

- Add User-Agent to zerus http usage

- build-std: Show output of rustup when needed, run `rustup` to get `rust-src` by automatically