Allow disabling the pip cache task

[EraYaN]

This helps a lot when you have reused and persistent self-hosted runners. The pip cache dir gets to be enourmous after a while slowing down this task.

[EnricoMi]

Maybe we can improve the caching and fix the growth of the cache. Do you know why it grows? Do you have a public repo that exhibits this behaviour?

[EraYaN]

I do not have a public repo sadly, it's a specific infrastructure problem. This is because it uses persistent self-hosted runners (so other pipeline steps use the same cache). This results in the cache task uploading the organisation wide cache, even though it is already persistent, so it just wastes time. It really needs to be disabled, improving it means not running it.

[EnricoMi]

So I presume your pipelines install other PIP packages, which all end up in the cache of this action.

This action should use an isolated cache, then there should be no such side effects. I'll look into this.

[EraYaN]

In general it is okay to just not use a cache at all, it will still be faster than using the cache.

[EnricoMi]

Fix #668 should improve your use case while continuing to work as it used to for other users.