Allow to skip downloading shellcheck if it is installed already

[SimonHeimberg]

Checklist

• [x] This Feature Request only contains 1 request (if you have multiple open multiple feature requests).

The idea

Allow to skip downloading shellcheck if it is installed already.

Implementation

• As an if condition on the step "Download shellcheck" (set by a new input or by a special value of "version")
• or probably better check in the bash script of this step if the command is available (maybe if "version" is (like) "-stable" and downloads "stable" if not available).

Alternatives

???

Additional context

shellcheck is installed on ubuntu runners: https://github.com/actions/runner-images/blob/ubuntu22/20230710.1/images/linux/Ubuntu2204-Readme.md#installed-apt-packages

[airtonix]

Would like this too as we already use asdf as our general setup step. so that handles installing shellcheck for us.