[BUGFIX] Correctly check stack in DownloadQueue

The extension manager's DownloadQueue does now properly check
the stack when adding extensions to the queue.

Resolves: #59975
Releases: 6.3, 6.2, 6.1
Change-Id: I90e9699608c6d0f3ebd44dd54eb0f471a4e082d9
Reviewed-on: https://review.typo3.org/31369
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters