In the absence of XCURSOR_PATH, the function XcursorLibraryPath
in libXcursor will return a hardcoded value unsuitable for NixOS.
Some desktops as well as display managers in NixOS currently do
set XCURSOR_PATH, but there are combinations where neither does
(e.g. SDDM+XFCE), resulting in no cursor themes being available.
The new definition if XCURSOR_PATH is effectively the same as what
KDE's startkde currently does.
Fixes issue #21442.
f80eeb5d85