Fix: Explicitly reference configuration for `friendsofphp/php-cs-fixer`

[localheinz]

What is the reason for this PR?

• [x] explicitly references the configuration for friendsofphp/php-cs-fixer

💁‍♂️ There is

• no rule in .gitignore that would ignore a .php-cs-fixer.php
• no need for a maintainer or contributor to override the existing configuration file .php-cs-fixer.dist.php

so why bother using the configuration resolving mechanism of friendsofphp/php-cs-fixer instead of explicitly referencing the configuration file with the configuration for our agreed-upon coding standard?

Also see #780.

Author's checklist

• [x] Follow the Contribution Guide

Summary of changes

Review checklist

• [x] All checks have passed
• [ ] Changes are added to the CHANGELOG.md
• [x] Changes are approved by maintainer

[localheinz]

Thank you, @bram-pkg!