system/nix: remove decomissioned builder, add repl-flake feature

repl-flake is required for nh

Author: fufexan

system/nix/builders.nix

@@ -6,7 +6,6 @@
   ...
 }: {
   imports = [
-    ./builders.nix
     ./nh.nix
     ./nixpkgs.nix
     ./substituters.nix
@@ -29,7 +28,7 @@
     settings = {
       auto-optimise-store = true;
       builders-use-substitutes = true;
-      experimental-features = ["nix-command" "flakes"];
+      experimental-features = ["nix-command" "flakes" "repl-flake"];
       flake-registry = "/etc/nix/registry.json";
 
       # for direnv GC roots