Disable the use of SWC, as it fails to download on trusted runners

We have an issue whereby the 'next build' step for the front-end fails when running on the trusted runners. This is because the built-in node fetch uses undici, which does not support the http_proxy environment variables until a more recent version than that included with growthbook.

This commit adds a custom babel configuration to the front-end, in order to prevent the use of swc and hopefully prevent the attempted download.

See https://nextjs.org/docs/messages/failed-loading-swc and https://phabricator.wikimedia.org/T365839#9998908

Bug: T365839