fix(rc): Prevent double-sourcing asdf when both native and Nix installs exist

rc.d/asdf tested native (~/.asdf) and Nix-profile asdf independently,
so if both were present on a machine, asdf would be sourced twice —
duplicate PATH entries and redefined completions/functions. Make the
Nix branch an elif so at most one is sourced.

rc.d/asdf

@@ -6,9 +6,7 @@ if [ -f ~/.asdf/asdf.sh ]; then
 	# activate completion
 	source ~/.asdf/completions/asdf.bash
 
-fi
-
-if [ -f "${HOME}/.nix-profile/share/asdf-vm/asdf.sh" ]; then
+elif [ -f "${HOME}/.nix-profile/share/asdf-vm/asdf.sh" ]; then
 	. "${HOME}/.nix-profile/share/asdf-vm/asdf.sh"
 	. "${HOME}/.nix-profile/share/bash-completion/completions/asdf.bash"
 fi