Add cached crate cargo-generate

[mciantyre]

Do the checklist before filing an issue:

• Is this related to the actions-rs Actions? If not, use GitHub Community forum to ask questions about Actions in a whole: https://github.community
• Does this crate helps with the Rust CI?
• Is it popular enough to be used by many users?

Crate

crates.io: https://crates.io/crates/cargo-generate
repository: https://github.com/ashleygwilliams/cargo-generate

Motivation

From the cargo-generate docs,

cargo-generate is a developer tool to help you get up and running quickly with a new Rust project by leveraging a pre-existing git repository as a template.

I'm a maintainer of a cargo-generate template. I use actions-rs/install to manually build cargo-generate. The build step is part of a larger job that validates the overall template, making sure that the template will work for end-users. It takes over 6 minutes to compile cargo-generate as part of the job (Ubuntu 18.04).

By providing cargo-generate in the tools cache, we can make it easier for cargo-generate template maintainers to test their templates.

[mciantyre]

On second thought, I don't think I can justify the many in "Is it popular enough to be used by many users?" A cached cargo-generate may only hold value for users who develop templates; it might not be useful for the Rust community at large.

I've unchecked the third box. I'm not sure this meets the criteria for tool cache inclusion.