nix: ci: dont test cuda and rocm (for now)

Until #6346 is resolved

flake.nix

@@ -168,9 +168,14 @@
  };
 
  # Packages exposed in `.#checks` will be built by the CI and by
- # `nix flake check`. Currently we expose all packages, but we could
- # make more granular choices
- checks = config.packages;
+ # `nix flake check`.
+ #
+ # We could test all outputs e.g. as `checks = confg.packages`.
+ #
+ # TODO: Build more once https://github.com/ggerganov/llama.cpp/issues/6346 has been addressed
+ checks = {
+ inherit (config.packages) default vulkan;
+ };
  };
  };
 }