Avoid downloading Gradle when version is pre-installed on runner

[bigdaz]

When setup-gradle is provided with a gradle-version input parameter, then this version is downloaded, installed and added to the PATH. If the required version of Gradle is already installed on the runner and on the PATH, then nothing needs to be done.

We should first check the version of Gradle that is pre-installed on the runner, and only provision Gradle when the pre-installed version does not match.