Update ISO URLs for NixOS 21.05

Updates the ISO URLs for 21.05 release of NixOS (make update).
Fixes error while building:

…
    virtualbox-iso: building the configuration in /mnt/etc/nixos/configuration.nix...
==> virtualbox-iso: error:
==> virtualbox-iso: Failed assertions:
==> virtualbox-iso: - Exactly one of users.users.vagrant.isSystemUser and users.users.vagrant.isNormalUser must be set.
==> virtualbox-iso:
==> virtualbox-iso: (use '--show-trace' to show detailed location information)
…

nix-community / nixbox

Update ISO URLs for NixOS 21.05 #71