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

tests/repl.sh

@@ -42,6 +42,11 @@ testRepl () {
     echo "$replOutput"
     echo "$replOutput" | grep -qs "while evaluating the file" \
       || fail "nix repl --show-trace doesn't show the trace"
+
+    nix repl "${nixArgs[@]}" --option pure-eval true 2>&1 <<< "builtins.currentSystem" \
+      | grep "attribute 'currentSystem' missing"
+    nix repl "${nixArgs[@]}" 2>&1 <<< "builtins.currentSystem" \
+      | grep "$(nix-instantiate --eval -E 'builtins.currentSystem')"
 }
 
 # Simple test, try building a drv