builders: don't accept flake configs

NixOS/nix#9649
https://gerrit.lix.systems/c/lix/+/1541

Author: mweinelt

builders/common/nix.nix

@@ -23,6 +23,7 @@
       };
 
     settings = {
+      accept-flake-config = false;
       builders-use-substitutes = true;
       extra-experimental-features = [
         "nix-command"