nixos/release.nix: More refactoring for multi-arch

Currently, even if you pass supportedSystems = [ "aarch64-linux" ] you
end up with e.g. `nixos.tests.docker` which actually silently runs on
x86_64-linux. Using the new callTestOnTheseSystems fixes that.

As a side-effect, this also causes a rename of
`nixos.tests.docker` -> `nixos.tests.docker.x86_64-linux`, which is IMHO
a good thing since it's makes them consistent with the rest of the
tests.