libxmlb: fix build

Tests failed with:
ERROR:../src/xb-self-test.c:462:xb_builder_ensure_watch_source_func: assertion failed (error == NULL)
cannot process file of type application/x-zerosize (g-io-error-quark, 15)

See: https://hydra.nixos.org/build/88774485

pkgs/development/libraries/libxmlb/default.nix

···
     "-Dgtkdoc=true"
   ];
 
+  preCheck = ''
+    export XDG_DATA_DIRS=$XDG_DATA_DIRS:${shared-mime-info}/share
+  '';
+
   doCheck = true;
 
   meta = with stdenv.lib; {