Guix commit e3d67b71cb0ccd5ad5563ebaffe6adc6c96c0c7c changes 'qtbase' to
point to 'qtbase-6'.  To allow for backward compatibility with Guix
1.3.0, use 'specification->package' to explicitly refer to qtbase@5 (we
cannot refer to the 'qtbase-5' variable since it didn't exist back
then).

* inria/staging.scm (S): New variable.
(paraview)[native-inputs, inputs]: Use it.
* inria/vite.scm (S): New variable.
(vite)[inputs, native-inputs]: Use it.