Record dependency graph file as GitHub Step output

Previously, this has been done in an init-script in the `gradle-build-action`.
However, the `GITHUB_OUTPUT` env var is different on every invocation, and reading this
value will invalidate the configuration cache. Moving it to the plugin should make the
plugin config-cache compatible.

plugin/src/main/kotlin/org/gradle/github/dependencygraph/GitHubDependencyGraphRenderer.kt

@@ -26,6 +26,11 @@ class GitHubDependencyGraphRenderer() : DependencyGraphRenderer {
         val outputFile = File(outputDirectory, "${snapshotParams.dependencyGraphJobCorrelator}.json")
 
         writeDependencySnapshot(snapshot, outputFile)
+
+        // Write the output file as a GitHub Actions step output
+        System.getenv("GITHUB_OUTPUT")?.let {
+            File(it).appendText("dependency-graph-file=${outputFile.absolutePath}\n")
+        }
     }
 
     private fun writeDependencySnapshot(graph: GitHubRepositorySnapshot, manifestFile: File) {