Configure Gradle User Home for dependency-graph

Instead of requiring an action step to generate the graph, configure Gradle User Home
so that subsequent Gradle invocations can generate a graph. Any generated graph files
are uploaded as artifacts on job completion.

- Construct job.correlator from workflow/job/matrix
- Export job.correlator as an environment var
- Upload artifacts at job completion in post-action step
- Specify the location of dependency graph report
- Only apply dependency graph init script when explicitly enabled

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

@@ -0,0 +1,6 @@
+buildscript {
+  dependencies {
+    classpath files("github-dependency-graph-gradle-plugin-0.0.3.jar")
+  }
+}
+apply plugin: org.gradle.github.GitHubDependencyGraphPlugin

src/resources/init-scripts/github-dependency-graph.init.gradle

@@ -1,7 +1,17 @@
-// TODO:DAZ This should be conditionally applied, since the script may be present when not required.
-initscript {
-  dependencies {
-    classpath files("github-dependency-graph-gradle-plugin-0.0.3.jar")
-  }
+if (System.env.GITHUB_DEPENDENCY_GRAPH_ENABLED != "true") {
+  return
 }
-apply plugin: org.gradle.github.GitHubDependencyGraphPlugin
+
+def reportDir = System.env.GITHUB_DEPENDENCY_GRAPH_REPORT_DIR
+def jobCorrelator = System.env.GITHUB_DEPENDENCY_GRAPH_JOB_CORRELATOR
+def reportFile = new File(reportDir, jobCorrelator + ".json")
+
+if (reportFile.exists()) {
+  println "::warning::No dependency report generated for step: report file for '${jobCorrelator}' created in earlier step. Each build invocation requires a unique job correlator: specify GITHUB_DEPENDENCY_GRAPH_JOB_CORRELATOR var for this step."
+  return
+}
+
+println "Generating dependency graph for '${jobCorrelator}'"
+
+// TODO:DAZ This should be conditionally applied, since the script may be present when not required.
+apply from: 'github-dependency-graph-gradle-plugin-apply.groovy'