Merge pull request #138623 from Ma27/bump-nixunstable

nixUnstable: 2.4pre20210908_3c56f62 -> 2.4pre20210922_bcd73eb

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

···
   nixUnstable = lib.lowPrio (callPackage common rec {
     pname = "nix";
     version = "2.4${suffix}";
-    suffix = "pre20210908_${lib.substring 0 7 src.rev}";
+    suffix = "pre20210922_${lib.substring 0 7 src.rev}";
 
     src = fetchFromGitHub {
       owner = "NixOS";
       repo = "nix";
-      rev = "3c56f62093601143838af923195f630d8ffae2d4";
-      sha256 = "sha256-pjdzLOEt8i1NQe4drLpp8+LrRd2MgsISSQEsMebz2kc=";
+      rev = "bcd73ebf60bb9ba6cb09f8df4366d5474c16e4a4";
+      sha256 = "sha256-wRbz8c22tlRn2/va/yOoLJijdJn+JJqLRDPRlifaEEA=";
     };
 
     boehmgc = boehmgc_nixUnstable;