[BUGFIX] Do not register storage factory slot during install

In install tool context TYPO3_MODE is set to BE
but there is no backend user initialized.
Because of that the storage permission aspect
fails as it requires a valid backend user object.

Do not register the slot when in install tool context.

Releases: 6.0, 6.1, 6.2
Resolves: #51733
Change-Id: I2da95ccd357b730b97bcf99bef21b18c04469274
Reviewed-on: https://review.typo3.org/23621
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel