Don't write the config file when --skip-config flag is given to the installer.

* scripts/installer_cli.php: Read the arguments list before checking
  if the config file is writeable.

scripts/install_cli.php

@@ -47,10 +47,10 @@ class CliInstaller extends Installer
      */
     function main()
     {
+        if ($this->prepare()) {
         if (!$this->checkPrereqs()) {
             return false;
         }
-        if ($this->prepare()) {
             return $this->handle();
        } else {
             $this->showHelp();