Allow configuring the root of the Rust project

[jonasbb]

The action currently assumes that the Rust project is directly in the root of the repository. This is not always the case, but Action cannot be configured with a working directory. Instead, a separate input is necessary. This was also requested as PROJECT_PATH for the audit-check action. actions-rs/audit-check#116

[faern]

Thank you @mbergkvist for adding this feature. Would it be possible to cut a release? I would love to use the file field in a CI job where we have multiple Rust workspaces in a single git repository, and would like to audit all of them.

[mbergkvist]

@jonasbb If you plan to make a new release, have a look at #56 and see if that can be included in the release too.

[jonasbb]

I released a new version and included #56 in it.

[faern]

Thank you for a very swift response and release! 🚀 This works really well for us!