feat: Handle http/s urls for legacy nix-shell command

[eureka-cpu]

If path starts with http:// or https://, it is interpreted as the URL of a tarball that will be downloaded and unpacked to a temporary location.
The tarball must include a single top-level directory containing at least a file named default.nix.

I think we can allow the nix errors to passthrough and just accept the url, which will automatically assume you want to use nix-shell since I don't think the nix shell command recognizes http/s tarball urls.

[eureka-cpu]

Ah, correction: nix shell expects a tarball containing a flake.nix, while nix-shell expects a tarball containing a default.nix.

[eureka-cpu]

This also makes me wonder if we should have a look at nix-shell -I for include paths.