When state is reused from the configuration cache, no dependencies are resolved.
This fix prevents the action from submitting an empty dependency graph in this case.
The repository URL used to resolve the `github-dependency-graph-gradle-plugin` is now
configurable, allowing a user to specify an internal proxy if the public portal is not available.
Specify a custom plugin repository using the `GRADLE_PLUGIN_REPOSITORY_URL` env var,
or the `gradle.plugin-repository.url` System property.
Fixes#933