gh-action: keep cache size constant

[sven-dmlr]

Situation

Currently the SecHub GH Action caches downloaded clients.
In case of "latest", regularly a new client will be downloaded.
So the cache size will increase forever

Wanted

Keep the cache lean and constant in size:

• cache only the latest downloaded version (clean cache directory before downloading a new client)
• after extraction delete the Files for other platforms than the current (e.g. keep linux-amd64 only)

[hamidonos]

Suggestion:

Extract the download from within the index.js code into a separate GitHub action step (action.yml)
the step checks for the version and the implements the cache based on the real version
that way the index.js does not have to do anything with caching -> better separation of concerns
Furthermore GitHub will take care of automatically removing the cache for a specific version if it's not used anymore