gdb: Fix infinite recursion

gdb/default.nix

@@ -3,15 +3,15 @@
 # Start `result/bin/gdb` and run `source result/gdbinit` once a Haskell executable is loaded.
 
 let
-  rev = "08d245eb31a3de0ad73719372190ce84c1bf3aee";
+  rev = "8a6e7487fb49a21edfa7abe9b6a054d71187698d";
   baseNixpkgs =
     builtins.fetchTarball {
     url = "https://github.com/NixOS/nixpkgs/archive/${rev}.tar.gz";
-    sha256 = "1g22f8r3l03753s67faja1r0dq0w88723kkfagskzg9xy3qs8yw8";
+    sha256 = "1cm57h8afry4cfng2jr3axyi945a64llx9kzfjjy5gniywm31mgn";
   };
-  nixpkgs = import baseNixpkgs {};
+  defaultNixpkgs = import baseNixpkgs {};
 
-in { nixpkgs ? nixpkgs }:
+in { nixpkgs ? defaultNixpkgs }:
 with nixpkgs; rec {
   ghc-gdb = python3Packages.buildPythonPackage {
     name = "ghc-gdb";