Cross-platform command to install dependencies

[mgeisler]

We should do something very simple to make Dustin's cargo xtask install-deps work. It could be as simple as a

fn main() -> Result<(), Box<dyn std::error::Error>> {
  Command::new(env!("CARGO")).arg("install").arg("mdbook").status()?;
  Command::new(env!("CARGO")).arg("install").arg("mdbook-i18n-helpers").status()?;
}

Put that in a new workspace member, hook up the Cargo alias and we should be in business 😄 To show that it works, make the GitHub CI code that install the dependencies use the new task.

Originally posted by @mgeisler in #2469 (comment)