themes: Fix linked only-child button style

Since `.linked > button:only-child` lacked `:dir(ltr|rtl)`,
`.linked > button:dir(ltr|rtl):last-child` style got greater precedence than the
direction-less `:only-child`.

Fixes #1294
5 jobs for fix/1294 in 20 minutes and 28 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #266776
fedora-x86_64

00:04:19

passed #266777
win32
msys2-mingw32

00:20:27

 
  Flatpak
manual #266778
allowed to fail manual
flatpak-manual:demo
manual #266780
allowed to fail manual
flatpak-manual:icon-browser
manual #266779
allowed to fail manual
flatpak-manual:widget-factory