Host singularity image and simply wget it for ubuntu

@yarikoptic I don't know if you were notified of this or not, but [the run last night of the Ubuntu build workflow](https://github.com/datalad/git-annex/actions/runs/395194053) failed with "Error getting image manifest using url https://www.singularity-hub.org/api/container/datalad/datalad-extensions:buildenv-git-annex-buster".

Start with the linked Ubuntu build workflow run and the failing Singularity image URL to confirm the manifest error. Determine where that workflow obtains the build environment and make the Ubuntu path retrieve the image successfully; the workflow should complete without the reported image-manifest failure.

Written by the indexing model from the issue text.