Build/Test Tools: Exclude `gutenberg` directory when running PHPCS. #564

Job	Run time
Determine Matrix	0s
${{ ((matrix.multisite && 'Multisite') \|\| 'Single Site') }} ${{ ((matrix.memcached && 'Memcached') \|\| 'Default') }}	0s
Failed workflow tasks	0s
matrix.label	0s
Slack Notifications	0s
	0s

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