Pass flake version to config version.

hosts/common/global/default.nix

@@ -11,6 +11,8 @@
   ];
 
   system.copySystemConfiguration = false; # not supported with flakes
+  # Let 'nixos-version --json' know about the Git revision of the flake
+  system.configurationRevision = nixpkgs.lib.mkIf (self ? rev) self.rev;
 
   # This value determines the NixOS release from which the default
   # settings for stateful data, like file locations and database versions