catch2_3: fix pkgsLLVM checkPhase (#458566)

pkgs/by-name/ca/catch2_3/package.nix

@@ -40,9 +40,9 @@ stdenv.mkDerivation rec {
     "-DCATCH_BUILD_TESTING=${if doCheck then "ON" else "OFF"}"
     "-DCATCH_ENABLE_WERROR=OFF"
   ]
-  ++ lib.optionals (stdenv.hostPlatform.isDarwin && doCheck) [
+  ++ lib.optionals (stdenv.cc.isClang && doCheck) [
     # test has a faulty path normalization technique that won't work in
-    # our darwin build environment https://github.com/catchorg/Catch2/issues/1691
+    # our darwin/LLVM build environment https://github.com/catchorg/Catch2/issues/1691
     "-DCMAKE_CTEST_ARGUMENTS=-E;ApprovalTests"
   ];