grsecurity module: permit chmod +s in sandboxed builds

While useless, some builds may dabble with setuid bits (e.g.,
util-linux), which breaks under grsec. In the interest of user
friendliness, we once again compromise by disabling an otherwise useful
feature ...

Closes https://github.com/NixOS/nixpkgs/issues/17501

nixos/modules/security/grsecurity.nix

···
       "kernel.grsecurity.chroot_deny_chroot" = mkForce 0;
       "kernel.grsecurity.chroot_deny_mount" = mkForce 0;
       "kernel.grsecurity.chroot_deny_pivot" = mkForce 0;
+      "kernel.grsecurity.chroot_deny_chmod" = mkForce 0;
     } // optionalAttrs containerSupportRequired {
       # chroot(2) restrictions that conflict with NixOS lightweight containers
       "kernel.grsecurity.chroot_deny_chmod" = mkForce 0;