[BUGFIX] Set position for alert container to fixed

In the install tool the flash message is not visible
when a user scrolls down. This has been solved by
setting position to fixed.

Resolves: #94479
Releases: master, 10.4
Change-Id: Id86c54bd7286567bd6fe95bb1aea21a9b4558fd6
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69714


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>