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/libutil/args.hh

@@ -25,6 +25,8 @@ public:
     /* Return a short one-line description of the command. */
     virtual std::string description() { return ""; }
 
+    virtual bool forceImpureByDefault() { return false; }
+
     /* Return documentation about this command, in Markdown format. */
     virtual std::string doc() { return ""; }