repl: --option pure-eval true actually enables pure eval mode

To quote Eelco in #5867:

> Unfortunately we can't do
>
>     evalSettings.pureEval.setDefault(false);
>
> because then we have to do the same in main.cc (where
> pureEval is set to true), and that would allow pure-eval
> to be disabled globally from nix.conf.

Instead, a command should specify that it should be impure by
default. Then, `evalSettings.pureEval` will be set to `false;` unless
it's overridden by e.g. a CLI flag.

In that case it's IMHO OK to be (theoretically) able to override
`pure-eval` via `nix.conf` because it doesn't have an effect on commands
where `forceImpureByDefault` returns `false` (i.e. everything where pure
eval actually matters).

Closes #5867

src/libcmd/repl.cc

@@ -1039,6 +1039,11 @@ struct CmdRepl : StoreCommand, MixEvalArgs
         });
     }
 
+    bool forceImpureByDefault() override
+    {
+        return true;
+    }
+
     std::string description() override
     {
         return "start an interactive environment for evaluating Nix expressions";
@@ -1053,8 +1058,6 @@ struct CmdRepl : StoreCommand, MixEvalArgs
 
     void run(ref<Store> store) override
     {
-        evalSettings.pureEval = false;
-
         auto evalState = make_ref<EvalState>(searchPath, store);
 
         auto repl = std::make_unique<NixRepl>(evalState);