[now-dev] Fix route normalization

[styfle]

This PR fixes a regression introduced in #3174 when removing the ^ and $ normalization.

The previous PR was normalizing user-defined routes but forgot to normalize builder routes.

This PR normalizes builder routes 👍

[TooTallNate]

Possible to add a test for this?

[styfle]

@TooTallNate I wrote a simple builder with a invalid regex route:

const { join, dirname } = require('path');
const { download, glob, shouldServe } = require('@now/build-utils');

async function build({ files, entrypoint, workPath, meta = {} }) {
  await download(files, workPath, meta);
  const mountpoint = dirname(entrypoint);
  const distPath = join(workPath, mountpoint);
  const routes = [{src: '(*.)', dest: '/index.html'}]; // broken regex
  const output = await glob('**', distPath, mountpoint);
  const watch = [join(mountpoint.replace(/^\.\/?/, ''), '**/*')];
  return { routes, output, watch };
};

module.exports =  { version: 2, build, shouldServe };

Then I added a now dev test fixture to ensure this builder fails with the correct error message.