Warn if the user is also using `actions/cache` directly?

[DilumAluthge]

A user may use both julia-actions/cache and actions/cache without realizing. Sometimes this may be intentional, but many times it may be accidental.

Can we detect if the user is also directly using actions/cache in the same job, and print a (non-fatal) warning if they are?

We can have an optional input that allows users to disable this warning; that way, if the user is indeed intentionally calling both julia-actions/cache and actions/cache, they can disable the warning and reduce the noise.

[IanButterworth]

In my experience in production setups it's fairly common to use both.