Using a nix store path for `xfpm-power-backlight-helper` and `xfce4-pm-helper` causes permission
errors after a rebuild when adjusting brightness or putting the system to sleep, because the
permitted executables are different from the ones `xfce4-power-manager` wants to execute. Using a
fixed path (/run/current-system/sw/bin) means the paths do not change after a rebuild, so there's no
permission error.
A similar approach is taken with gnome's `gsd-backlight-helper`.
Fixes#77485
6a2999310e