Exclude build/ from the PHP compatibility scan

A stale build/ left by composer zip is gitignored output, but phpcompat still
scanned it, so composer test could fail on a copy of the plugin rather than on
the source. phpcs already excluded it; this makes the two agree.

Author: ajaydsouza

composer.json

@@ -33,7 +33,7 @@
 		],
 		"phpcompat": [
 			"@phpcompat:setup",
-			"phpcompat-tools/vendor/bin/phpcs -p --standard=PHPCompatibility --runtime-set testVersion 7.4-8.6 --extensions=php --ignore=*/vendor/*,*/node_modules/*,*/phpcompat-tools/*,*/_site/* ."
+			"phpcompat-tools/vendor/bin/phpcs -p --standard=PHPCompatibility --runtime-set testVersion 7.4-8.6 --extensions=php --ignore=*/build/*,*/vendor/*,*/node_modules/*,*/phpcompat-tools/*,*/_site/* ."
 		],
 		"phpcompat:clean": "rm -rf phpcompat-tools",
 		"test": [