flake.nix: Handle missing rev attribute

Dirty trees no longer set 'rev', so we need to handle this.

flake.nix

@@ -23,8 +23,8 @@
           import ./nixos/lib/eval-config.nix (args // {
             modules = modules ++
               [ { system.nixos.versionSuffix =
-                    ".${lib.substring 0 8 self.lastModified}.${self.shortRev}";
-                  system.nixos.revision = self.rev;
+                    ".${lib.substring 0 8 self.lastModified}.${self.shortRev or "dirty"}";
+                  system.nixos.revision = lib.mkIf (self ? rev) self.rev;
                 }
               ];
           });