nixUnstable: patch boehmgc (nix#4944)

As has been done in https://github.com/NixOS/nix/pull/4944

This introduces the boehmgc_nix and boehmgc_nixUnstable attributes
which are useful for external packages that link with Nix and its
boehmgc.

(cherry picked from commit 596ac242af767b38e48af9076887b206dc72e472)

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

@@ -2,7 +2,8 @@
 , storeDir ? "/nix/store"
 , stateDir ? "/nix/var"
 , confDir ? "/etc"
-, boehmgc
+, boehmgc_nix
+, boehmgc_nixUnstable
 , Security
 }:
 
@@ -205,7 +206,9 @@ in rec {
       sha256 = "sha256-cToMnZU3+UpjeiiXnG3clz9zn8Xk+TbB7UbqmLMrlFk=";
     };
 
-    inherit storeDir stateDir confDir boehmgc;
+    boehmgc = boehmgc_nix;
+
+    inherit storeDir stateDir confDir;
   });
 
   nixUnstable = lib.lowPrio (callPackage common rec {
@@ -220,7 +223,9 @@ in rec {
       sha256 = "sha256-ARRiLrDOK+JQtvVXsYegspENYimQzilvdTfO7eiBuaA=";
     };
 
-    inherit storeDir stateDir confDir boehmgc;
+    boehmgc = boehmgc_nixUnstable;
+
+    inherit storeDir stateDir confDir;
 
   });