chore(3p): Bump nixos-unstable channel

This bumps the channel to a commit that includes fixes for an nginx
issue we have been seeing:

https://github.com/NixOS/nixpkgs/pull/95264

Includes the following compatibility fixes:

- tests disabled in third_party.bufbuild: These were enabled
  unexpectedly by the update, but don't run in the sandbox because
  they want to download things from github

Change-Id: I98a3b5de57f62f1fd3a37701fa1896eddeedff85
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1759
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: tazjin <mail@tazj.in>

third_party/bufbuild/default.nix

@@ -14,6 +14,8 @@ pkgs.buildGoModule {
     sha256 = "1rni5swfnb4sbrd9rls4mc3902xhqrlsja96lfcdfjzx08g6kg20";
   };
 
+  doCheck = false;
+
   # TODO(riking): postinstall produce shell completions for bash, fish, zsh
   # bin/buf bash-completion
   # bin/buf zsh-completion