[TASK] Tune php-cs-fixer setup

Some minor php-cs-fixer modifications:
* Move main config file to Build/php-cs-fixer
* Adapt runTests.sh setup and helper scripts
* Check Build/ directory, too
* Simplify config a bit

Change-Id: I74bea96791c9b74331030bd7c9c26d566ce6c90c
Resolves: #98525
Releases: main, 11.5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75984


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>