Fix useMoldLinker when cross compiling

After #400734 we can make useMoldLinker work when cross compiling.
We need the following:
- Need to use pkgs.buildPackages.mold so that we use mold binaries
  for the build machine instead of the host.
- Need to create unprefixed ld.mold wrapper because gcc will not look
  for a triple-prefixed ld.mold when passing -fuse-ld=mold.

Tested on an aarch64 machine with this change + #400734:

$ nix-build -K --expr 'with import "${./.}" { crossSystem = { config = "x86_64-unknown-linux-gnu"; }; }; hello.override { stdenv = useMoldLinker stdenv; }' && strings result/bin/hello | grep mold
/nix/store/w1yg0am7iyqsgi253h84lsp92178sgfi-hello-x86_64-unknown-linux-gnu-2.12.1
mold 2.37.1 (compatible with GNU ld)

pkgs/stdenv/adapters.nix

@@ -350,8 +350,9 @@ rec {
       let
         bintools = stdenv.cc.bintools.override {
           extraBuildCommands = ''
-            wrap ${stdenv.cc.bintools.targetPrefix}ld.mold ${../build-support/bintools-wrapper/ld-wrapper.sh} ${pkgs.mold}/bin/ld.mold
-            wrap ${stdenv.cc.bintools.targetPrefix}ld ${../build-support/bintools-wrapper/ld-wrapper.sh} ${pkgs.mold}/bin/ld.mold
+            wrap ld.mold ${../build-support/bintools-wrapper/ld-wrapper.sh} ${pkgs.buildPackages.mold}/bin/ld.mold
+            wrap ${stdenv.cc.bintools.targetPrefix}ld.mold ${../build-support/bintools-wrapper/ld-wrapper.sh} ${pkgs.buildPackages.mold}/bin/ld.mold
+            wrap ${stdenv.cc.bintools.targetPrefix}ld ${../build-support/bintools-wrapper/ld-wrapper.sh} ${pkgs.buildPackages.mold}/bin/ld.mold
           '';
         };
       in