support cairo-trace (but it doesn't work)

this env var is needed for cairo-trace to run, but note it doesn't
actually print anything. I don't know why but will speculate that
it's a gir thing

pkgs/maps/default.nix

@@ -67,6 +67,7 @@ in stdenv.mkDerivation {
   ];
   GIO_EXTRA_MODULES = [ "${glib-networking.out}/lib/gio/modules" ];
   RXI_JSON="${rxi-json}/";
+  CAIRO_TRACE_SO = "${cairo.out}/lib/cairo/libcairo-trace.so";
 
   makeFlags = [ "PREFIX=${placeholder "out"}" "NAME=${pname}" ];