[BUGFIX] Install System Extensions in update wizard fails

In the update wizard it isn't possible to install any system extension
because the PackageManager isn't initialized. During bootstrap only the
class FilesafePackageManager is loaded and registered. This patch
registers the initialized FailsafePackageManager as singleton instance.

Resolves: #53708
Releases: 6.2
Change-Id: I9eb9ab8ac1fd655fd642e386cd51895f1d8c1e21
Reviewed-on: https://review.typo3.org/25467
Reviewed-by: Thomas Maroschik
Tested-by: Thomas Maroschik
Reviewed-by: Steffen Müller
Tested-by: Steffen Müller