anchor.diff:  
     toolchainName = "com.apple.dt.toolchain.XcodeDefault";
     sdkName = "${xcodePlatform}${sdkVer}";
  +  xcrunSdkName = lib.toLower xcodePlatform;
   
     # TODO: expose MACOSX_DEPLOYMENT_TARGET in nix so we can use it here.
     sdkBuildVersion = "17E189";
  
positive.code:     esac
      shift
  done
  if ! [[ -z "$@" ]]; then
     exec "$@"
  fi
  
negative.code:   '';
  
    xcrun = writeShellScriptBin "xcrun" ''
  args=( "$@" )
  
  # If an SDK was requested, check that it matches.
  for ((i = 0; i < ''${#args[@]}; i++)); do
    case "''${args[i]}" in
      --sdk | -sdk)
        i=$((i + 1))
        if [[ "''${args[i]}" != '${xcrunSdkName}' ]]; then
          echo >&2 "xcodebuild: error: SDK \"''${args[i]}\" cannot be located."
          exit 1
        fi
        ;;
    esac
  done
  
  while [ $# -gt 0 ]; do
     case "$1" in
           --sdk | -sdk) shift ;;
  