[FrameworkBundle] Prevent silenced warning by checking if file exists

Author: GromNaN

src/Symfony/Bundle/FrameworkBundle/Command/CacheClearCommand.php

@@ -200,7 +200,7 @@ private function isNfs(string $dir): bool
 
         if (null === $mounts) {
             $mounts = [];
-            if ('/' === \DIRECTORY_SEPARATOR && $files = @file('/proc/mounts')) {
+            if ('/' === \DIRECTORY_SEPARATOR && is_readable('/proc/mounts') && $files = @file('/proc/mounts')) {
                 foreach ($files as $mount) {
                     $mount = \array_slice(explode(' ', $mount), 1, -3);
                     if (!\in_array(array_pop($mount), ['vboxsf', 'nfs'])) {