Add a flag to install a crate in debug mode

[sunshowers]

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Motivation

Hi there, and thanks for writing this action! I have some tools from repositories and crates.io I'd like to install in my actions, and they tend to compile a lot faster in debug mode than in release mode.

Could you add a debug: true flag to the action to make it easy to add new crates in debug mode? You could make it incompatible with the use-toolchain option.

Additional context

The tools I'd like to install are:
https://github.com/facebookincubator/cargo-guppy (from the git repo)
https://crates.io/crates/cargo-readme