mc: move signature breaking sed from post to pre fixup

Commit cb0e81b5b6 reverted changes from a7f0bf7916,
which are required to have mc working on aarch64-darwin,
where binary signatures are enforced.

pkgs/tools/misc/mc/default.nix

@@ -57,10 +57,12 @@ stdenv.mkDerivation rec {
       --replace /bin/rm ${coreutils}/bin/rm
   '';
 
-  postFixup = ''
+  preFixup = ''
     # remove unwanted build-dependency references
     sed -i -e "s!PKG_CONFIG_PATH=''${PKG_CONFIG_PATH}!PKG_CONFIG_PATH=$(echo "$PKG_CONFIG_PATH" | sed -e 's/./0/g')!" $out/bin/mc
-  '' + lib.optionalString (!stdenv.isDarwin) ''
+  '';
+
+  postFixup = lib.optionalString (!stdenv.isDarwin) ''
     # libX11.so is loaded dynamically so autopatch doesn't detect it
     patchelf \
       --add-needed ${libX11}/lib/libX11.so \