Apply fix from #611 to nix-tools (#612)

Author: hamishmack

overlays/haskell.nix

@@ -2,7 +2,26 @@
 #
 # for hygenic reasons we'll use haskell-nix as a prefix.
 # Using haskell.nix in nix is awkward as I needs to be quoted.
-final: prev: {
+final: prev:
+let
+  # Why `final.buildPackages.buildPackages.git`?
+  #
+  # It turns out `git` depends on `gdb` in a round about way:
+  #  git -> openssh -> libfido2 -> systemd -> python libxml -> Cython -> gdb
+  # Somewhere in that chain there should perhaps be a `buildPackages` so
+  # that the `gdb` that is used is not the one for debugging code in
+  # the `final` (but instead the one for debugging code in
+  # `final.buildPackages`).
+  #
+  # Using `final.buildPackages.git` causes two problems:
+  #
+  #   * Multiple versions of `git` (and that dependency chain
+  #     to `gdb` are needed when cross compiling).
+  #   * When `gdb` does not exist for `js`, so when cross
+  #     compiling with ghcjs `final.buildPackages.git` fails
+  #     to build at all.
+  inherit (final.buildPackages.buildPackages) git nix-prefetch-git;
+in {
     haskell-nix = with final.haskell-nix; {
 
         # Default modules, these will always be included.
@@ -60,21 +79,7 @@ final: prev: {
         # Utility functions for working with the component builder.
         haskellLib = let hl = import ../lib {
             inherit (final) stdenv lib runCommand recurseIntoAttrs srcOnly;
-            # It turns out `git` depends on `gdb` in a round about way:
-            #  git -> openssh -> libfido2 -> systemd -> python libxml -> Cython -> gdb
-            # Somewhere in that chain there should perhaps be a `buildPackages` so
-            # that the `gdb` that is used is not the one for debugging code in
-            # the `final` (but instead the one for debugging code in
-            # `final.buildPackages`).
-            #
-            # Using `final.buildPackages.git` causes two problems:
-            #
-            #   * Multiple versions of `git` (and that dependency chain
-            #     to `gdb` are needed when cross compiling).
-            #   * When `gdb` does not exist for `js`, so when cross
-            #     compiling with ghcjs `final.buildPackages.git` fails
-            #     to build at all.
-            inherit (final.buildPackages.buildPackages) git;
+            inherit git;
             haskellLib = hl;
         }; in hl;
 
@@ -180,8 +185,8 @@ final: prev: {
         # We probably never want to actually cross compile nix-tools on
         # it's own.
         nix-tools-cross-compiled = final.lib.makeOverridable (import ../nix-tools) {
-            inherit (final) pkgs lib symlinkJoin makeWrapper
-                           git nix nix-prefetch-git;
+            inherit (final) pkgs lib symlinkJoin makeWrapper nix;
+            inherit git nix-prefetch-git;
             inherit (final.haskell-nix) fetchExternal cleanSourceHaskell mkCabalProjectPkgSet;
             hpack = null; # nix-tools does not use hpack project files
         };