zfs-kernel: fix strip in cross-compilation

pkgs/os-specific/linux/zfs/default.nix

···
       # Since zfs compress kernel modules on installation, our strip hooks skip stripping them.
       # Hence we strip modules prior to compression.
       postBuild = optionalString buildKernel ''
-         find . -name "*.ko" -print0 | xargs -0 -P$NIX_BUILD_CORES strip --strip-debug
+         find . -name "*.ko" -print0 | xargs -0 -P$NIX_BUILD_CORES ${stdenv.cc.targetPrefix}strip --strip-debug
       '';
 
       postInstall = optionalString buildKernel ''