message: [cli] Remove `.vercel/output` during `vc build --output`
  The `.vercel/output` directory gets wiped away when `--output` is _not_ being used, but it should be deleted even when the flag is being used so that build scripts targeting Build Output API directly always start with a fresh slate.
diff: @@ -212,10 +212,15 @@ export default async function main(client: Client): Promise<number> {
  }
  // Delete output directory from potential previous build
  + const defaultOutputDir = join(cwd, OUTPUT_DIR);
  const outputDir = argv['--output']
  ? resolve(argv['--output'])
  - : join(cwd, OUTPUT_DIR);
  - await fs.remove(outputDir);
  + : defaultOutputDir;
  + await Promise.all([
  + fs.remove(outputDir),
  + // Also delete `.vercel/output`, in case the script is targeting Build Output API directly
  + outputDir !== defaultOutputDir ? fs.remove(defaultOutputDir) : undefined,
  + ]);
  const buildsJson: BuildsManifest = {
  '//': 'This file was generated by the `vercel build` command. It is not part of the Build Output API.',
  