nixos/system-environment: prepend wrapperDir to PATH (#70430)

This fixes user environment setup for sessions which doesn't successfully go
through a shell init.

Note we don't go through `sessionVariables` as we want the wrappers to have
highest priority. It would also cause wrapperDir to occur twice when in shell
sessions, as shells use `sessionVariables` too while prepending wrapperDir in a
custom snippet.

In particular logging in and out of gnome-shell could result in a broken path
without this fix.