metadata/install-qa-check.d/08gentoo-paths: add guix

sys-apps/nix and sys-apps/guix are twins:
- sys-apps/nix manages /nix/store
- sys-apps/guix manages /gnu/store

Added 'gnu' and mentioned packages that own the directory.

Bug: https://bugs.gentoo.org/670902
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

metadata/install-qa-check.d/08gentoo-paths

@@ -16,7 +16,9 @@ gentoo_path_check() {
 	# /home is not included as no ebuilds should install files there
 	local allowed_paths_toplevel=(
 		"${allowed_common_dirs[@]}"
-		boot dev etc nix opt srv usr var
+		boot dev etc opt srv usr var
+		nix # sys-apps/nix, bug #670902
+		gnu # sys-apps/guix, bug #670902
 	)
 
 	# directories in /usr which can be installed to by ebuilds