nix copy and nix-copy-closure works inconsistently on nixbuild.net

[rickynils]

The following works:

• nix-copy-closure --to eu.nixbuild.net
• nix copy --to ssh-ng://eu.nixbuild.net
• nix copy --from ssh-ng://eu.nixbuild.net

The following does not work:

• nix-copy-closure --from eu.nixbuild.net
• nix copy --from ssh://eu.nixbuild.net

So, you can copy both to and from nixbuild.net but you have to select the right combination of commands and protocols. We should fix this so all combinations work.

[rickynils]

Most of the closure copying scenarios have now been fixed, but this is still not working: nix copy --from ssh://eu.nixbuild.net. This issue will be kept open until that use case has been fixed.