conduit: fix reproducibility by not leaking kernel version

Override CMAKE_SYSTEM_VERSION to prevent the build kernel version
from being embedded in the build output. This fixes non-reproducible
builds where CONDUIT_SYSTEM_TYPE would vary between builds
(e.g., "Linux-6.12.47" vs "Linux-6.16.9").

The fix follows the same approach used in Qt6 packages to ensure
reproducible builds across different build environments.

Fixes: #450434

pkgs/by-name/co/conduit/package.nix

@@ -36,6 +36,8 @@ stdenv.mkDerivation (finalAttrs: {
   ];
 
   cmakeFlags = [
+    # Don't leak kernel version into the build output for reproducibility
+    "-DCMAKE_SYSTEM_VERSION="
     (lib.cmakeBool "ENABLE_MPI" mpiSupport)
   ];