Add nix flake

[hrenard]

I used this template.
It helps to use this cli in nix ci. (example)

[azmeuk]

Thank you for the contribution!
Is it possible for you to add a GHA step that builds the flake so we are sure this does not break during future developments?

[azmeuk]

@hrenard for the record, all scim-* packages except scim-cli are proposed to be integrated in nixpgks
NixOS/nixpkgs#370603

[azmeuk]

Thank you!