[TASK] Ensure proper gitlab ci configuration for branch 12.4

Adjustes gitlab workflow configurations to match branch
`12.4` testing. This includes dedicated cache keys and
setting to not run after merge on branch `12.4`.

Additionally, setting `COMPOSER_ROOT_VERSION` environment
variable is reintroduced to `Build/Scripts/runTests.sh`.

That ensures, that composer can detect the correct root
version.

Resolves: #100800
Releases: 12.4
Change-Id: I640ee609c28e53f6c1ede468c2629214d1a52187
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78930


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>