[5.2] Set php-cs-fixer to run on multiple cores

[Hackwar]

Summary of Changes

php-cs-fixer allows to be run on multiple cores since of late. I actually don't know if this even works in drone, but if it works in either drone or locally, it should speed up its run and make life a little bit easier.

Testing Instructions

Check that php-cs-fixer still runs in drone.

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[brianteeman]

I have tested this item ✅ successfully on 2918c03

Running analysis on 4 cores with 10 files per process.

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[Hackwar]

I'm setting this PR to RTC, since we don't need tests for our drone setup and I plan on merging this as soon as 5.2.0 alpha 3 has been released on tuesday.

[laoneo]

Thanks!