ci: Pin php-cs-fixer/shim

Signed-off-by: Julius Härtl <jus@bitgrid.net>

Author: juliusknorr

composer.json

@@ -4,6 +4,7 @@
 		"nextcloud/coding-standard": "^1.0",
 		"squizlabs/php_codesniffer": "^3",
 		"phan/phan": "^5",
+		"php-cs-fixer/shim": "3.20.0",
 		"guzzlehttp/guzzle": "^7",
 		"staabm/annotate-pull-request-from-checkstyle": "^1.1.0"
 	},