Add a cache-save input to support restore-only dependency caching

[naokihaba]

Context

Currently, cache: true enables both cache restoration in the main action and cache saving in the post action.

In many setups, it is desirable to restore caches on every branch and PR, but only write new caches from a default or trusted branch like main or develop.

Right now, to achieve this, you either have to disable caching on PRs altogether (losing restoration entirely) or manually wire up actions/cache/restore and actions/cache/save to mimic setup-vp's internal cache paths and key generation. The latter is brittle because it duplicates internal logic and easily drifts over time.

Proposal

It would be great to add an optional cache-save input (defaulting to true to avoid breaking changes).

- uses: voidzero-dev/setup-vp@v1.18.0
  with:
    cache: true
    cache-save: ${{ github.ref == 'refs/heads/develop' }}

The matrix would look like this:

cache	cache-save	Restore	Save
false	Any value	No	No
true	Omitted or true	Yes	Yes
true	false	Yes	No

Essentially, the post action would only save the cache when both cache and cache-save are enabled.

For reference, mise-action implements a similar option:
https://github.com/jdx/mise-action/blob/0c6ea298a49e2fc9b6b8ec854c2e6c292490f032/action.yml#L55-L58