nixos/release.nix: Use forTheseSystems from release-lib

Currently, even if you pass `supportedSystems = [ "aarch64-linux" ]` you
end up with e.g. `nixos.iso_graphical.x86_64-linux` job. Using
forTheseSystems from release-lib avoids that.

This shouldn't affect the usual x86 trunk-combined jobset.