Kali purple: install kali-system-gui (instead of core)

The dep chain is kali-system-gui > kali-system-cli > kali-system-core.

kali-system-cli contains wget and curl, which we probably want to have,
and will fix https://gitlab.com/kalilinux/kali-purple/documentation/-/issues/9

I'm not sure about kali-system-gui, but I have the impression that it
was the intention to install it in Kali Purple, but at some point it was
forgotten and we install kali-system-core instead.

kali-config/installer-purple/packages

@@ -4,7 +4,7 @@
 # You can customize the set of Kali metapackages (groups of tools) available
 # in the installer ISO.
 # For the complete list see: https://tools.kali.org/kali-metapackages
-kali-system-core
+kali-system-gui
 kali-themes-purple  # temporary, until we do it a better way
 
 # Tools