Use tools cache

[fwilhe]

Currently the action downloads the piper binary newly each time it is invoked. Github offers a "tools cache" which we could use.

cf https://github.com/actions/toolkit/tree/master/packages/tool-cache

[CCFenner]

This is intended for reuse on the same workflow or across workflows?

[fwilhe]

Finally, you can cache these directories in our tool-cache. This is useful if you want to switch back and forth between versions of a tool, or save a tool between runs for private runners (private runners are still in development but are on the roadmap).

My understanding is that this "only" caches in one run, but might still be useful if we're building workflows that use it often. But on the other hand I don't see a real need as of now as the downloads are really fast, so probable we won't gain much here. More "nice to have" imo.