edk2-uefi-shell: fix cross compilation

This PYTHON_COMMAND workaround was introduced in #201095, but it does
not seem to have caused issues so far, until the version update in
bef3f40f05. Not sure why this was not an
issue before this update and why it is now, but I think this fix makes
sense regardless of the exact root cause, and I don't think it's worth
the effort to dig further.

pkgs/by-name/ed/edk2-uefi-shell/package.nix

@@ -6,6 +6,7 @@
   util-linux,
   nasm,
   python3,
+  pkgsBuildHost,
 }:
 edk2.mkDerivation "ShellPkg/ShellPkg.dsc" (finalAttrs: {
   pname = "edk2-uefi-shell";
@@ -32,7 +33,7 @@ edk2.mkDerivation "ShellPkg/ShellPkg.dsc" (finalAttrs: {
 
   # Set explicitly to use Python 3 from nixpkgs. Otherwise, the build system will detect and try to
   # use `/usr/bin/python3` on Darwin when sandboxing is disabled.
-  PYTHON_COMMAND = "${lib.getBin python3}/bin/python3";
+  PYTHON_COMMAND = "${lib.getBin pkgsBuildHost.python3}/bin/python3";
 
   # We only have a .efi file in $out which shouldn't be patched or stripped
   dontPatchELF = true;