Add a warning when trying to run integrity:check-core on git

Signed-off-by: Joas Schilling <coding@schilljs.com>

core/Command/Integrity/CheckCore.php

@@ -61,6 +61,11 @@ class CheckCore extends Base {
 	 * {@inheritdoc }
 	 */
 	protected function execute(InputInterface $input, OutputInterface $output): int {
+		if (!$this->checker->isCodeCheckEnforced()) {
+			$output->writeln('<comment>integrity:check-core can not be used on git checkouts</comment>');
+			return 2;
+		}
+
 		$result = $this->checker->verifyCoreSignature();
 		$this->writeArrayInOutputFormat($input, $output, $result);
 		if (count($result) > 0) {