Advantages of including the SHA256:
(i) removes the impurity of the tarball contents being changed
(ii) if sha256 is not included, then each nixops deploy triggers a re-download of the tarball
Here's how to get the expected hash:
$ nix-prefetch-url --unpack 'https://gitlab.com/simple-nixos-mailserver/nixos-mailserver/-/archive/v2.2.0/nixos-mailserver-v2.2.0.tar.gz'
unpacking...
[0.0 MiB DL]
path is '/nix/store/dwg8xlfnlw7mhr4cjk1viwmm0b249b74-nixos-mailserver-v2.2.0.tar.gz'
0gqzgy50hgb5zmdjiffaqp277a68564vflfpjvk1gv6079zahksc
