When supplying a url to nix-shell:

nix-shell https://github/<owner>/<repo>/archive/master.tar.gz

It does not attempt to load the shell.nix file contained in the tarball, instead only loading default.nix, which fails if it doesn't provide a viable shell.

I would expect the behavior to be consistent with launching from the base of a repository, namely, load shell.nix if it exists, default.nix otherwise.