Honor PANAMA_PATH in the two stages that clobbered it

install and link-dotfiles treat it as a default; install-packages and
change-settings overwrote an exported override, so a clone anywhere but
~/.local/share/Panama sourced the extras catalog from a path that does
not exist and copied config/copy from the wrong tree.

setup/scripts/install-packages

@@ -23,7 +23,10 @@ packages_in() {
 }
 
 # --- Defined Paths ---
-PANAMA_PATH="$HOME/.local/share/Panama"
+# The default, not an assignment: ./install and link-dotfiles honor an exported
+# PANAMA_PATH, and clobbering it here made a clone anywhere else source the
+# extras catalog from a path that does not exist.
+PANAMA_PATH="${PANAMA_PATH:-$HOME/.local/share/Panama}"
 
 # Reading the extras catalog, shared with `panama apps` so the two front doors
 # cannot disagree about what a category contains.