Allow action, wrapper and testKit to share downloaded Gradle distributions

[bigdaz]

Currently, there are 3 mechanisms by which Gradle distributions may be downloaded:

1. When the gradle-build-action provisions a specified Gradle version for use
2. When the Gradle wrapper is used to run a project
3. When Gradle TestKit is used to test a project with different Gradle versions.

Ideally, these would all share a common download location and cache entry, avoiding the need to re-download the same Gradle distribution multiple times.

In addition, if a Gradle distribution is pre-installed on the runner, it would be good if this version didn't have to be re-downloaded for the 3 mechanisms listed above.

[bigdaz]

This comment indicates that TestKit downloads are not being properly cached, exacerbating the issue.