When Nix build is finished and pushed to Cachix, update a specific branch.
When Nix build is finished and pushed to Cachix, update a specific branch.

This should allow CI jobs that use our Cachix to only test against Coq versions
that are already available in Cachix, avoiding very long build and time outs e.g.
in the math-classes CI.

Co-authored-by: Vincent Laporte <5b9500d3ac195ca166d1abfbb700bb9943ca7b7b@inria.fr>

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