nixUnstable: 2.4pre20210922 -> 2.4pre20211001

Contains a fix for lowdown 0.9.0.

pkgs/tools/package-management/nix/default.nix

···
   nixUnstable = lib.lowPrio (callPackage common rec {
     pname = "nix";
     version = "2.4${suffix}";
-    suffix = "pre20210922_${lib.substring 0 7 src.rev}";
+    suffix = "pre20211001_${lib.substring 0 7 src.rev}";
 
     src = fetchFromGitHub {
       owner = "NixOS";
       repo = "nix";
-      rev = "bcd73ebf60bb9ba6cb09f8df4366d5474c16e4a4";
-      sha256 = "sha256-wRbz8c22tlRn2/va/yOoLJijdJn+JJqLRDPRlifaEEA=";
+      rev = "4f496150eb4e0012914c11f0a3ff4df2412b1d09";
+      sha256 = "00hxxk66f068588ymv60ygib6vgk7c97s9yia3qd561679rq3nsj";
     };
 
     boehmgc = boehmgc_nixUnstable;