`cache-saved` output for `cache/save`

[rikhuijzer]

cache-hit exists for cache/restore and provides whether there was a cache hit. However, when using cache/save, there is no similar output which can be used to check for whether the cache was saved. This could be useful when building composite actions and checking whether the action was triggered? Feel free to close this issue if it isn't considered useful.

[IanButterworth]

I agree this would be helpful