fix: unstable=toolkit-lib-engine has no effect (#668)

The `engine` field is not being passed on when passing the call to the
worker pool.

---
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Author: rix0rrr

packages/@aws-cdk/integ-runner/lib/workers/integ-test-worker.ts

@@ -136,6 +136,7 @@ export async function runIntegrationTestsInParallel(
         dryRun: options.dryRun,
         verbosity: options.verbosity,
         updateWorkflow: options.updateWorkflow,
+        engine: options.engine,
       }], {
         on: printResults,
       });