Create php-memory-limits.ini only one time

Change how the php-memory-limits.ini is managed because when changed between PHP versions all the configuration added previously is overrided and it a waste of time if you work with several PHP versions during the day

cli/Valet/PhpFpm.php

@@ -82,10 +82,12 @@ public function createConfigurationFiles(string $phpVersion): void
         $destDir = dirname(dirname($fpmConfigFile)).'/conf.d';
         $this->files->ensureDirExists($destDir, user());
 
+        if (!file_exists($destDir.'/php-memory-limits.ini')) {
             $this->files->putAsUser(
                 $destDir.'/php-memory-limits.ini',
                 $this->files->getStub('php-memory-limits.ini')
             );
+        }
 
         $contents = str_replace(
             ['VALET_USER', 'VALET_HOME_PATH'],