Regression: self.rev is absent

[brprice]

I am using flakes with nixops, and employ the idiom

system.configurationRevision =
  if self ? rev
  then self.rev
  else throw "Refusing to build from a dirty Git tree!";

so I can trace which configuration a running deployment is using, via nixos-version --json (and enforce that every deploy corresponds to a commit).

Unfortunately this broke in the state backend rework in #1264. Now self.rev never exists, even in a clean checkout.

Reproducer:
Have a git repo with a flake.nix containing (contents are unimportant, except for testing self.rev)

{
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-21.11";

  outputs = { self, nixpkgs }:
{
      nixopsConfigurations.default = {
        network.description = "A server";
        network.storage.legacy.databasefile = "/tmp/state.nixops";
        nixpkgs = nixpkgs;
        serverA = {config, pkgs, ...}: {
          # let nixos-version --json know about the git revision of this flake
          system.configurationRevision =
            if self ? rev
            then self.rev
            else throw "Refusing to build from a dirty git tree!";
          fileSystems."/" = {
            device = "/dev/sda1";
            fsType = "ext3";
          };
          boot.loader.grub = {
            enable = true;
            version = 2;
            device = "/dev/sda";
          };
        };
      };
    };
}

Note that old versions of nixops work fine (this is just before #1264):

rm /tmp/state.nixops
NIXOPS_STATE=/tmp/state.nixops nix shell -L -v github:NixOS/nixops?rev=b55cbb23cc92f14b0e028ff2f8327cab4d9af5c6 -c nixops create --flake . -d serverA
NIXOPS_STATE=/tmp/state.nixops nix shell -L -v github:NixOS/nixops?rev=b55cbb23cc92f14b0e028ff2f8327cab4d9af5c6 -c nixops deploy -d serverA --build-only

However, newer versions complain that the flake is dirty -- in actuality the checkout is clean, but self.rev does not exist.
Post-#1264:

NIXOPS_STATE=/tmp/state.nixops nix shell -L -v github:NixOS/nixops?rev=9b3ac919d87db872854fb528064821b959ac38c1 -c nixops deploy -d serverA --build-only

master:

NIXOPS_STATE=/tmp/state.nixops nix shell -L -v github:NixOS/nixops?rev=ce1c57ec104636954296c229468bb65fa4ec7f8d -c nixops deploy -d serverA --build-only