nixpkgs/elvish: place XDG_DATA_DIRS first

Placing it after causes any nested elvish's to use the old code instead.

.config/nixpkgs/elvish/flake.nix

@@ -56,7 +56,7 @@
         in
         writeScriptBin "elvish" ''
           #!/bin/sh
-          export XDG_DATA_DIRS="$XDG_DATA_DIRS:${config}"
+          export XDG_DATA_DIRS="${config}:$XDG_DATA_DIRS"
           export PATH="$PATH:${carapace-bin}/bin"
           if [ -z "$KEYCTL_INIT" ]; then
             export KEYCTL_INIT=1