In the v2.6 branch of the mongodb/mongo repo, etc/evergreen.yml file has fetch and put artifacts functions using ${build_variant} var as the folder in the mciuploads bucket.
It should be using mongodb-mongo-v2.6 folder and ${build_variant} as a subfolder instead, like the rest of the branches.
- command: s3.put params: aws_key: ${aws_key} aws_secret: ${aws_secret} local_file: target.tgz remote_file: ${build_variant}/${build_id}.tgz bucket: mciuploads permissions: public-read content_type: application/tar display_name: Artifacts
"fetch artifacts" : command: s3.get params: aws_key: ${aws_key} aws_secret: ${aws_secret} remote_file: ${build_variant}/${build_id}.tgz bucket: mciuploads extract_to: src
It should be
remote_file: mongodb-mongo-v2.6/${build_variant}/${build_id}.tgz
in both cases.