How to use the @now/build-utils/fs/run-user-scripts.js.runNpmInstall function in @now/build-utils

console.log(`using default ${tsconfig}`)
    fs.outputJsonSync(ftsTsConfigPath, {
      compilerOptions: {
        target: 'es2015',
        moduleResolution: 'node'
      }
    })
  }

  // TODO: for local testing purposes
  // console.log('linking dependencies for fts...')
  // execa.shellSync('yarn link fts fts-http', { cwd: ftsPath, stdio: 'inherit' })

  console.log()
  console.log('installing dependencies for fts...')
  await runNpmInstall(ftsPath, npmArguments)

  return [downloadedFiles, nccPath, ftsPath, entrypointFsDirname]
}

const downloadedFiles = await download(filesWithoutLockfiles, workPath);

    console.log('normalizing package.json');
    const packageJson = normalizePackageJson(
        await readPackageJson(downloadedFiles),
    );
    console.log('normalized package.json result: ', packageJson);
    await writePackageJson(workPath, packageJson);

    if (process.env.NPM_AUTH_TOKEN) {
        console.log('found NPM_AUTH_TOKEN in environment, creating .npmrc');
        await writeNpmRc(workPath, process.env.NPM_AUTH_TOKEN);
    }

    console.log('running npm install...');
    await runNpmInstall(workPath, ['--prefer-offline']);
    console.log('running user script...');
    await runPackageJsonScript(workPath, 'now-build');
    console.log('running npm install --production...');
    await runNpmInstall(workPath, ['--prefer-offline', '--production']);
    if (process.env.NPM_AUTH_TOKEN) {
        await unlink(path.join(workPath, '.npmrc'));
    }

    const filesAfterBuild = await glob('**', workPath);

    console.log('preparing lambda files...');
    const dotNuxtServerRootFiles = await glob('.nuxt/dist/*', workPath);
    const nodeModules = excludeFiles(
        await glob('node_modules/**', workPath),
        file => file.startsWith('node_modules/.cache'),
    );

async function downloadInstallAndBundle(
  { files, entrypoint, workPath },
  { npmArguments = [] } = {}
) {
  const userPath = path.join(workPath, 'user')
  const nccPath = path.join(workPath, 'ncc')
  const ftsPath = path.join(workPath, 'fts')

  console.log()
  console.log('downloading user files...')
  const downloadedFiles = await download(files, userPath)

  console.log()
  console.log("installing dependencies for user's code...")
  const entrypointFsDirname = path.join(userPath, path.dirname(entrypoint))
  await runNpmInstall(entrypointFsDirname, npmArguments)

  console.log('writing ncc package.json...')
  fs.outputJsonSync(path.join(nccPath, 'package.json'), {
    dependencies: {
      '@zeit/ncc': '0.11.0',
      typescript: '3.2.4'
    }
  })

  console.log()
  console.log('installing dependencies for ncc...')
  await runNpmInstall(nccPath, npmArguments)

  console.log('writing fts package.json...')
  fs.outputJsonSync(path.join(ftsPath, 'package.json'), {
    dependencies: {

filesOnlyEntryDirectory,
        entryDirectory,
    );
    const filesWithoutLockfiles = excludeLockFiles(filesWithEntryDirectoryRoot);
    await download(filesWithoutLockfiles, workPath);
    await download(await glob('.nuxt/**', workPath), cachePath);
    await download(await glob('node_modules/**', workPath), cachePath);

    console.log('.nuxt folder contents', await glob('.nuxt/**', cachePath));
    console.log(
        '.cache folder contents',
        await glob('node_modules/.cache/**', cachePath),
    );

    console.log('running npm install...');
    await runNpmInstall(cachePath);

    return {
        ...(await glob('.nuxt/server/index.spa.html', cachePath)),
        ...(await glob('.nuxt/server/index.ssr.html', cachePath)),
        ...(await glob('.nuxt/server/vue-ssr-client-manifest.json', cachePath)),
        ...(await glob('.nuxt/server/server-bundle.json', cachePath)),
        ...(await glob('node_modules/**', cachePath)),
        ...(await glob('yarn.lock', cachePath)),
    };
};

async function installDependencies(files, workPath) {
  const hasPkgJSON = Boolean(files['package.json']);
  if (hasPkgJSON) {
    console.log('Installing dependencies...');
    await runNpmInstall(workPath, ['--prefer-offline']);
  } else {
    throw new Error('Missing package.json');
  }
}

console.log()
  console.log("installing dependencies for user's code...")
  const entrypointFsDirname = path.join(userPath, path.dirname(entrypoint))
  await runNpmInstall(entrypointFsDirname, npmArguments)

  console.log('writing ncc package.json...')
  fs.outputJsonSync(path.join(nccPath, 'package.json'), {
    dependencies: {
      '@zeit/ncc': '0.11.0',
      typescript: '3.2.4'
    }
  })

  console.log()
  console.log('installing dependencies for ncc...')
  await runNpmInstall(nccPath, npmArguments)

  console.log('writing fts package.json...')
  fs.outputJsonSync(path.join(ftsPath, 'package.json'), {
    dependencies: {
      fts: 'latest',
      'fts-http': 'latest'
    }
  })

  const ftsTsConfigPath = path.join(ftsPath, tsconfig)
  if (downloadedFiles[tsconfig]) {
    console.log(`copying ${tsconfig}`)
    fs.copySync(downloadedFiles[tsconfig].fsPath, ftsTsConfigPath)
  } else {
    console.log(`using default ${tsconfig}`)
    fs.outputJsonSync(ftsTsConfigPath, {