xdist workers don't have access to the API key and cannot fetch the
manifest. Prior to this change, the xdist worker would attempt to
fetch the manifest if the fetch attempt in the parent process failed.
This change fixes the bug so that xdist workers will never attempt
to fetch the manifest, even if the fetch in the parent process failed.

AWS Lambda limits payloads to 6MB, which puts an upper bound
on the size of the Unflakable backend API's request bodies. This
change leverages a newer backend API endpoint to upload test runs
to presigned S3 URLs, which are not subject to AWS Lambda limits.
The backend then retrieves the upload directly from S3.