[BUGFIX] Follow-Up: ext:install Core update

Fix a small mistake in variable usage during creating of the
download directory.

Resolves: #52547
Releases: 6.2
Change-Id: I080d43edfe9d6921d0ccca28a34f42bdb788f630
Reviewed-on: https://review.typo3.org/24476
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe