Support LIGHTHOUSE_BUILDER_HOST env variable in frontend server

[arnellebalane]

Currently the frontend server is hardcoded to use the builder at https://builder-dot-lighthouse-ci.appspot.com, which is not customizable (except editing the code directly) for when running a custom instance of the builder.

This PR makes the URL to the builder configurable through the LIGHTHOUSE_BUILDER_HOST environment variable.

# in lighthousebot/frontend
LIGHTHOUSE_BUILDER_HOST=https://my-custom-builder.com yarn start