How to use the @now/build-utils.runNpmInstall function in @now/build-utils

async function installPhp({ workPath, config }) {
  console.log(`🐘 Installing PHP ${configuration.getVersion(config)} lib.`);

  // Install defined PHP version on the fly into the tmp folder
  const packageJson = {
    dependencies: {
      [configuration.getPhpNpm(config)]: 'canary',
    }
  };

  const packageJsonPath = path.join(workPath, 'package.json');
  await writeFile(packageJsonPath, JSON.stringify(packageJson));

  await runNpmInstall(path.dirname(packageJsonPath), [
    '--prod',
    '--prefer-offline',
  ]);

  console.log(`🐘 Installing PHP ${configuration.getVersion(config)} lib OK.`);
}

workPath,
  config,
  meta,
}: DownloadOptions) {
  const downloadedFiles = await download(files, workPath, meta);

  console.log('Installing dependencies...');
  const installTime = Date.now();
  const entrypointFsDirname = join(workPath, dirname(entrypoint));
  const nodeVersion = await getNodeVersion(
    entrypointFsDirname,
    undefined,
    config
  );
  const spawnOpts = getSpawnOptions(meta, nodeVersion);
  await runNpmInstall(
    entrypointFsDirname,
    ['--prefer-offline'],
    spawnOpts,
    meta
  );
  debug(`Install complete [${Date.now() - installTime}ms]`);

  const entrypointPath = downloadedFiles[entrypoint].fsPath;
  return { entrypointPath, entrypointFsDirname, nodeVersion, spawnOpts };
}

);
    pkg.scripts = {
      'now-build': 'next build',
      ...(pkg.scripts || {}),
    };
    shouldRunScript = 'now-build';
    await writePackageJson(entryPath, pkg);
  }

  if (process.env.NPM_AUTH_TOKEN) {
    debug('Found NPM_AUTH_TOKEN in environment, creating .npmrc');
    await writeNpmRc(entryPath, process.env.NPM_AUTH_TOKEN);
  }

  console.log('Installing dependencies...');
  await runNpmInstall(entryPath, ['--prefer-offline'], spawnOpts, meta);

  let realNextVersion: string | undefined;
  try {
    realNextVersion = require(resolveFrom(entryPath, 'next/package.json'))
      .version;

    debug(`Detected Next.js version: ${realNextVersion}`);
  } catch (_ignored) {
    debug(`Could not identify real Next.js version, that's OK!`);
  }

  if (!isLegacy) {
    await createServerlessConfig(workPath, entryPath, realNextVersion);
  }

  debug('Running user script...');