Fix sizebot not working due to missing auth token (#26423)

Sizebot works by fetching the base artifacts from CI. CircleCI recently
updated this endpoint to require an auth token. This is a problem for PR
branches, where sizebot runs, because we don't want to leak the token to
arbitrary code written by an outside contributor.

This only affects PR branches. CI workflows that run on the main branch
are allowed to access environment variables, because only those with
push access can land code in main.

As a temporary workaround, we'll fetch the assets from a mirror,
react-builds.vercel.app. This is the same app that hosts the sizebot
diff previews.

Need to figure out a longer term solution. Perhaps by converting sizebot
into a proper GitHub app.