Guix packaging: allow jami-qt to run on non-Mesa systems

Guix applications cannot currently run on drivers not part of Mesa, such as the nvidia proprietary drivers or AMD ones.

To be truly universal, Guix applications should be able to run on such systems as well.

This basically requires fixing Mesa in Guix to use glvnd (see the upstream issue here: https://issues.guix.gnu.org/49847).

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