Add authorization header to artifacts request (#24106)

* Add authorization header to artifacts request

CircleCI's artifacts API was updated; it now errors unless you're
logged in. This affects any of our workflows that download
build artifacts.

To fix, I added an authorization header to the request.

* Update sizbot to pull artifacts from public mirror

We can't use the normal download-build script in sizebot because it
depends on the CircleCI artifacts API, which was recently changed to
require authorization. And we can't pass an authorization token
without possibly leaking it to the public, since we run sizebot on
PRs from external contributors. As a temporary workaround, this job
will pull the artifacts from a public mirror that I set up. But we
should find some other solution so we don't have to maintain
the mirror.