libcmd/repl: Fix missing runNix in repl

Without this :u, :sh and :i repl commands fail with:

> Cannot run 'nix-shell'/`nix-env` because no method of calling the Nix
> CLI was provided. This is a configuration problem pertaining to how
> this program was built.

Remove the default ctor argument as it evidently makes catching
refactoring bugs much harder. `NixRepl` implementation lives completely
in `repl.cc`, so we can be as explicit as necessary.

src/libcmd/repl.cc

@@ -124,7 +124,7 @@ std::string removeWhitespace(std::string s)
 
 
 NixRepl::NixRepl(const LookupPath & lookupPath, nix::ref<Store> store, ref<EvalState> state,
-            std::function<NixRepl::AnnotatedValues()> getValues, RunNix * runNix = nullptr)
+            std::function<NixRepl::AnnotatedValues()> getValues, RunNix * runNix)
     : AbstractNixRepl(state)
     , debugTraceIndex(0)
     , getValues(getValues)
@@ -841,7 +841,8 @@ std::unique_ptr<AbstractNixRepl> AbstractNixRepl::create(
         lookupPath,
         std::move(store),
         state,
-        getValues
+        getValues,
+        runNix
     );
 }
 
@@ -859,7 +860,8 @@ ReplExitStatus AbstractNixRepl::runSimple(
             lookupPath,
             openStore(),
             evalState,
-            getValues
+            getValues,
+            /*runNix=*/nullptr
         );
 
     repl->initEnv();