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)