fix: remove experimental feature flag repl-flake

this was removed in upstream nix.

Author: soopyc

nix/common.nix

@@ -14,7 +14,7 @@
       "t2linux.cachix.org-1:P733c5Gt1qTcxsm+Bae0renWnT8OLs0u9+yfaK2Bejw="
     ];
 
-    experimental-features = [ "nix-command" "flakes" "repl-flake" ];
+    experimental-features = [ "nix-command" "flakes" ];
   };
 
   environment.systemPackages = with pkgs; [