xsltproc: Make "-" read from stdin
e7210659 fixes openconnect#1 (closed). But there's a (similar) problem with xsltproc here: https://gitlab.gnome.org/GNOME/libxslt/-/issues/109.
Upstream already committed a fix for that, but I'm not seeing that in the cherry-picked 1.1.40-2: https://gitlab.gnome.org/GNOME/libxslt/-/commit/16f3d7c4954da41788a46e0b14baff2c5785d314
HTH