appimage-run: add xdg-user-dirs for flutter apps (#350962)

Changed files
+1
pkgs
build-support
appimage
+1
pkgs/build-support/appimage/default.nix
···
xorg.xrandr
which
perl
xdg-utils
iana-etc
krb5
···
xorg.xrandr
which
perl
+
xdg-user-dirs # flutter desktop apps
xdg-utils
iana-etc
krb5