confinement: fix assert for serviceConfig.ProtectSystem

serviceConfig.ProtectSystem is usually a string so if set, the assert
itself would error out leaving no useable trace:

  # nixos-rebuild switch --show-trace
  building Nix...
  building the system configuration...
  error: while evaluating the attribute 'config.system.build.toplevel' at /nix/var/nix/profiles/per-user/root/channels/nixos/nixos/modules/system/activation/top-level.nix:293:5:
  while evaluating 'foldr' at /nix/var/nix/profiles/per-user/root/channels/nixos/lib/lists.nix:52:20, called from /nix/var/nix/profiles/per-user/root/channels/nixos/nixos/modules/system/activation/top-level.nix:128:12:
  while evaluating 'fold'' at /nix/var/nix/profiles/per-user/root/channels/nixos/lib/lists.nix:55:15, called from /nix/var/nix/profiles/per-user/root/channels/nixos/lib/lists.nix:59:8:
  while evaluating anonymous function at /nix/var/nix/profiles/per-user/root/channels/nixos/nixos/modules/system/activation/top-level.nix:121:50, called from undefined position:
  while evaluating the attribute 'assertion' at /nix/var/nix/profiles/per-user/root/channels/nixos/nixos/modules/security/systemd-confinement.nix:163:7:
  value is a string while a Boolean was expected

Fix the check to give a sensible assert message instead; the attribute
should either be not set or false bool to pass.

nixos/modules/security/systemd-confinement.nix

@@ -160,7 +160,7 @@ in {
               + " the 'users.users' option instead as this combination is"
               + " currently not supported.";
     }
-    { assertion = !cfg.serviceConfig.ProtectSystem or false;
+    { assertion = cfg.serviceConfig ? ProtectSystem -> cfg.serviceConfig.ProtectSystem == false;
       message = "${whatOpt "ProtectSystem"}. ProtectSystem is not compatible"
               + " with service confinement as it fails to remount /usr within"
               + " our chroot. Please disable the option.";