The xdg-open utility is only ever a runtime dependency and its
dependents only expect that it accept a URI as a command line
argument and do something with it that the user would expect.
For such as a trivial relationship it should be possible for
users to override xdg-open with something else in their PATH.
fixes 'ValueError: Namespace Gtk not available'
since 71cbf3b809 gobject-introspection's
hook looks for gir files in buildInputs instead of nativeBuildInputs
