Add example for installing a cargo subcommand.

[dvc94ch]

There seems to be some support for caching the subcommand, and just running cargo install manually doesn't install it to a location that is on the PATH.

[svartalf]

Hi, @dvc94ch! Can you elaborate more on this issue?

~/.cargo/bin should be added into the PATH during the actions-rs/toolchain execution, so as long as you are using it, any cargo install-ed binary will be available in your PATH.

Yet, proper caching of the installed binary is not implemented yet, see actions-rs/meta#21

EDIT: here is a wild example of cargo install cargo-nono and the following call of it: https://github.com/svartalf/rust-macaddr/runs/371402468

[dvc94ch]

Looks like I made a mistake. It seems that it was missing a tool that the cargo subcommand needed.