Cache pkl cache 🙃

[StefMa]

When you run pkl and is has to download dependencies it will save it in ~/.pkl/cache.
Maybe we want to store these dependencies in cache instead of re-downloading them all the time.

A cache key could be the PklProject file as this contains the dependencies property... 🤔
Additionally to the pkl-version of course.

[jasongwartz]

This is a great idea, and could be implemented using the @actions/cache toolkit.

PRs welcome 😄