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

+4
+4
pkgs/development/libraries/libxmlb/default.nix
··· 22 22 "-Dgtkdoc=true" 23 23 ]; 24 24 25 + preCheck = '' 26 + export XDG_DATA_DIRS=$XDG_DATA_DIRS:${shared-mime-info}/share 27 + ''; 28 + 25 29 doCheck = true; 26 30 27 31 meta = with stdenv.lib; {