On Nix, the datadir option is returning just share, which causes issues when
the program tries to load the data:

$ result/bin/gupnp-network-light
** (gupnp-network-light:29089): CRITICAL **: 20:12:47.920: Unable to load the XML description file: Failed to parse share/gupnp-tools/xml/network-light-desc.xml

Let’s fix it to always use absolute paths.