`if: always()` with `cache/save` may lead to errors (fix README!)

[andreasabel]

https://github.com/actions/cache/blob/04f198bf0b2a39f7230a4304bf07747a0bddf146/save/README.md?plain=1#L83-L84

In my experience, if we had an exact cache hit, the attempt to save the cache (under the same key) will produce an error. So, the docs should be updated to something like

if: always() && !steps.<cache-restore-step>.outputs.cache-hit

at least in cases where we did a restore.

As cache-hit will only be true if the cache-primary-key equals the cache-matched-key this will always save the cache when it is possible.

[github-actions[bot]]

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[andreasabel]

Issue is still unfixed: https://github.com/actions/cache/blob/704facf57e6136b1bc63b828d79edcd491f0ee84/save/README.md?plain=1#L83-L84

[andreasabel]

ATTN: @bethanyj28 Please triage/fix.