fix: restore depGraph assignment used by assertions in plugin.spec.ts

The eslint migration accidentally removed the variable assignment for
depGraph in a test that still uses it in assertions, causing a
ReferenceError at runtime.

Author: parker-snyk

test/system/plugin.spec.ts

@@ -128,7 +128,7 @@ describe("plugin", () => {
 
     test("should successfully scan a local image loaded from a tar archive", async () => {
       const pluginResult = await plugin.scan({ path: imageNameWithTag });
-      pluginResult.scanResults[0].facts.find(
+      const depGraph: DepGraph = pluginResult.scanResults[0].facts.find(
         (fact) => fact.type === "depGraph",
       )!.data;