feat: add cache-targets input propagated to Swatinem/rust-cache

[somaz94]

Adds a cache-targets input that is forwarded to Swatinem/rust-cache as cache-targets, mirroring the existing cache-all-crates / cache-workspace-crates / cache-save-if passthroughs.

This lets you skip caching the workspace target directory (e.g. when using sccache) without disabling rust-cache entirely. The default is true, identical to rust-cache's own default, so behavior is unchanged when the input is left unset.

Resolves #84.

Validation:

• pre-commit run --files action.yml README.md CHANGELOG.md passes locally, including Validate GitHub Actions (action.yml schema) and check-yaml.
• Default value true matches Swatinem/rust-cache's default, so the rendered cache step is unchanged unless the input is explicitly set.

[jonasbb]

Thanks, the code looks all good. I will merge this and tag a new version