[BUGFIX] Available extension list not updated on CLI

On installing a manually downloaded extension usign the
extension:install CLI API, the list of available extensions does not
get updated prior to the installation, causing it to fail.

Resolves: #53071
Releases: 6.2
Change-Id: I8610f27b8fe36c15f02d88e7b4275099e5443b96
Reviewed-on: https://review.typo3.org/24972
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer