nixpkgs/neovim: set XDG_RUNTIME_DIR to TMPDIR if present

nix-shell overwrites TMPDIR so we want XDG_RUNTIME_DIR to be set so it's
consistent inside and outside.

.config/nixpkgs/elvish/flake.nix

@@ -57,6 +57,9 @@
               export KEYCTL_INIT=1
               keyctl new_session >/dev/null
             fi
+            if [ -z "$XDG_RUNTIME_DIR" ] && [ -n "$TMPDIR" ]; then
+              export XDG_RUNTIME_DIR="$TMPDIR"
+            fi
             exec ${elvish}/bin/elvish -rc "${config}/elvish/rc.elv" "$@"
           '';