libvirt: disable util-linux on darwin

pkgs/development/libraries/libvirt/default.nix

@@ -141,11 +141,13 @@ stdenv.mkDerivation rec {
     # delete only the first occurrence of this
     sed -i '0,/qemuxml2argvtest/{/qemuxml2argvtest/d;}' tests/meson.build
 
+  '' + lib.optionalString isLinux ''
     for binary in mount umount mkfs; do
       substituteInPlace meson.build \
         --replace "find_program('$binary'" "find_program('${lib.getBin util-linux}/bin/$binary'"
     done
 
+  '' + ''
     substituteInPlace meson.build \
       --replace "'dbus-daemon'," "'${lib.getBin dbus}/bin/dbus-daemon',"
   '' + lib.optionalString isLinux ''