`cache-workspace-crates` should include `hashFiles('rust/**/*.rs')` in cache key (or a configurable pattern)

[thomaseizinger]

I am using the cache-workspace-crates feature to cache very expensive release builds of crates. In order for that to actually work, I need to incorporate the hash of all source files into the cache key, otherwise the cache doesn't get updated as soon as any of the Rust code changes.

I can incorporate the key myself using the key input but that would mean, no caches are restored as soon as any of the Rust code changes.

Are you open to a PR that adds another input for configuring this pattern or adding the hashFiles bit by default if cache-workspace-crates is set to true?