[BUGFIX] Avoid double slash in cache directory

When configuring a different cache directory,
the setter is called in SimpleFileBackend, which tries
to add the cache identifier as last directory name.

At this time however the cache identifier is still empty
and the full directory path is composed in setCache anyway.

Resolves: #88961
Releases: 8.7, 9.5, master
Change-Id: I44c99d78e98d951d83666ff51eeea37b90a52302
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/61499


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Richard Haeser <richard@maxserv.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Richard Haeser <richard@maxserv.com>