xcfe4-genmon-plugin cannot execute the dig command

If a dig command is configured to be displayed by the generic monitor, like this:

dig gitlab.xfce.org

an error is shown:

dig: ./src/unix/core.c:570: uv__close: Assertion fd > STDERR_FILENO' failed.`

This doesn't seem to be a problem in dig itself, but a problem in the way xfce-genmon-plugin handles the file descriptors (see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1000793 for some discussion).