Plugin repository URL is configurable

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

src/resources/init-scripts/gradle-build-action.github-dependency-graph-gradle-plugin-apply.groovy

@@ -1,6 +1,12 @@
 buildscript {
+  def getInputParam = { String name ->
+      def envVarName = name.toUpperCase().replace('.', '_').replace('-', '_')
+      return System.getProperty(name) ?: System.getenv(envVarName)
+  }
+  def pluginRepositoryUrl = getInputParam('gradle.plugin-repository.url') ?: 'https://plugins.gradle.org/m2'
+
   repositories {
-    maven { url "https://plugins.gradle.org/m2/" }
+    maven { url pluginRepositoryUrl }
   }
   dependencies {
     classpath "org.gradle:github-dependency-graph-gradle-plugin:1.0.0"