nix-cli: use pure/restricted eval for help pages

This avoids any complications that can arise from the environment
affecting evaluation of the help pages (which don't need to be calling
out to anything external anyways)

A recent example of one of these problems is
https://github.com/NixOS/nix/issues/14085, which would break help pages
by causing them to make invalid calls to the dummy store they're
evaluated with

Fixes: https://github.com/NixOS/nix/issues/14062
Co-authored-by: Sergei Zimmerman <sergei@zimmerman.foo>

src/nix/main.cc

@@ -226,8 +226,8 @@ static void showHelp(std::vector<std::string> subcommand, NixArgs & toplevel)
 
     auto mdName = subcommand.empty() ? "nix" : fmt("nix3-%s", concatStringsSep("-", subcommand));
 
-    evalSettings.restrictEval = false;
-    evalSettings.pureEval = false;
+    evalSettings.restrictEval = true;
+    evalSettings.pureEval = true;
     EvalState state({}, openStore("dummy://"), fetchSettings, evalSettings);
 
     auto vGenerateManpage = state.allocValue();