Improve cache-summary reporting when Gradle User Home already exists

[bigdaz]

When the action detects an existing Gradle User Home, it will not attempt to save/restore the Gradle User Home to the GitHub Actions cache. This manifests in a strange way in the caching summary:

Instead, the summary should state that caching was disabled due to a pre-existing Gradle User Home. It might also be useful to suggest disabling other forms of Gradle User Home caching, like cache: gradle from the setup-java action.