Disable Cachix pushes (#3312)

* Disable Cachix pushes

This is not safe until we have sandboxed builds. For TGI alone
this might not be a huge issue, but with Cachix caching disabled
in hf-nix, TGI CI would build all the packages and push it to
our cache.

* fix: bump docs

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Co-authored-by: drbh <david.richard.holtz@gmail.com>