Auto merge of #6128 - cswindle:install-registry, r=alexcrichton
Auto merge of #6128 - cswindle:install-registry, r=alexcrichton

Allow registry option for cargo install

This allows installing rust crates directly from cargo install by passing --registry to the command.

rustyhorde/cargo,rust-lang/cargo,rustyhorde/cargo,rust-lang/cargo,nipunn1313/cargo,rust-lang/cargo,tshepang/cargo,tshepang/cargo,vojtechkral/cargo,tshepang/cargo,tsurai/cargo,vojtechkral/cargo,eminence/cargo,vojtechkral/cargo,nipunn1313/cargo,eminence/cargo,tshepang/cargo,nipunn1313/cargo,tsurai/cargo,eminence/cargo,rustyhorde/cargo,vojtechkral/cargo,tsurai/cargo,rustyhorde/cargo,rust-lang/cargo,nipunn1313/cargo,tsurai/cargo,tsurai/cargo,eminence/cargo