nix: clarify the meaning of "broken" and "badPlatforms"

.devops/nix/package.nix

@@ -178,7 +178,15 @@ effectiveStdenv.mkDerivation (
     };
 
     meta = {
-      broken = (useCuda && effectiveStdenv.isDarwin) || (useMetalKit && !effectiveStdenv.isDarwin);
+      # Configurations we don't want even the CI to evaluate. Results in the
+      # "unsupported platform" messages. This is mostly a no-op, because
+      # cudaPackages would've refused to evaluate anyway.
+      badPlatforms = optionals (useCuda || useOpenCL) lib.platforms.darwin;
+
+      # Configurations that are known to result in build failures. Can be
+      # overridden by importing Nixpkgs with `allowBroken = true`.
+      broken = (useMetalKit && !effectiveStdenv.isDarwin);
+
       description = "Inference of LLaMA model in pure C/C++${descriptionSuffix}";
       homepage = "https://github.com/ggerganov/llama.cpp/";
       license = lib.licenses.mit;
@@ -201,7 +209,8 @@ effectiveStdenv.mkDerivation (
         SomeoneSerge
       ];
 
-      platforms = lib.platforms.unix;
+      # Extend `badPlatforms` instead
+      platforms = lib.platforms.all;
     };
   }
 )