cleanup(devkit): prevent mutating targetDefaults in migration to crys…

…tal helper (#26316)

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Fixes #

packages/devkit/src/generators/plugin-migrations/executor-to-plugin-migrator.ts

@@ -347,8 +347,9 @@ class ExecutorToPluginMigrator<T> {
   }
 
   #getTargetDefaultsForExecutor() {
-    this.#targetDefaultsForExecutor =
-      this.#nxJson.targetDefaults?.[this.#executor];
+    this.#targetDefaultsForExecutor = structuredClone(
+      this.#nxJson.targetDefaults?.[this.#executor]
+    );
   }
 
   #getCreatedTargetForProjectRoot(targetName: string, projectRoot: string) {