bug#52393: pdf links not clickable when zathura is launched via xdg-open

["bdju" via Bug reports for GNU Guix <bug-guix@gnu.org>]

guix (GNU Guix) f199427c1b6dd8e3428e25d4e15f604b3c90a3b7
Guix System

works: zathura file.pdf
doesn't work: xdg-open file.pdf

Both launch zathura since it's my default reader. In the xdg-open case,
clicking a link in a pdf seems to not do anything. If I launched with
zathura explicitly, it opens the link in my existing instance of
qutebrowser as I would expect.
No errors related to clicking the link seem to show up in a terminal.
When launched either way I do get a lot of errors saying
```
error: plugin: filetype already registered: application/pdf
```
and then similar ones for oxps, epub+zip, xml, etc. as well as a message
that libdjvu.so cannot be registered as a plugin. These seem like
non-fatal errors and are probably unrelated, but I thought I would
mention them anyway.