activation scripts: proper file permissions

zfs.nix

@@ -12,6 +12,8 @@ in
       #!/bin/sh
       rm -fr ${zfs-key} || true
       cp ${./secrets/zfs-key} ${zfs-key}
+      chmod 0500 ${zfs-key}
+      chown root:wheel ${zfs-key}
     '';
   };