Feat: Control whether caches are written

In some cases you want to read but not save a new cache. For instance when you run workflows on a tag action, then you probably want to read caches from the main/default branch, but you most likely do not want to create new caches. The current solution is to separate restore and save actions which is cumbersome.

In https://github.com/actions/cache/pull/1452 the always_save feature was removed.

I propose to add a new config save-on-success that defaults to true and can be set false to prevent cache writing. Now this new value can be set from available context - just like you could when separating the steps - granted the decision must be available upfront. That above described decision is in fact available immediately on job creation. So we can use the new setting to simplify the setup.

Start by reviewing the cache action's current restore/save flow and the removal of the always_save feature described in PR 1452. Determine how the proposed save-on-success setting should be supplied from available context. Done means the setting defaults to true and false prevents new cache writes while preserving cache reads.