flake: do not throw on dirty tree

This is mostly inconvenient, I have enough discipline not to need it
anymore.

flake.nix

@@ -19,10 +19,7 @@
       defaultModules = [
         ({ ... }: {
           # Let 'nixos-version --json' know about the Git revision
-          system.configurationRevision =
-            if self ? rev
-            then self.rev
-            else throw "Refusing to build from a dirty Git tree!";
+          system.configurationRevision = self.rev or "dirty";
         })
         {
           nixpkgs.overlays = (lib.attrValues self.overlays) ++ [