flake.nixConfig: fix flake-registry config settings

Before this commit, nixConfig.flake-registry didn't have any real effect
on the evaluation, since config was applied after inputs were evaluated.
Change this behavior: apply the config in the beginning of flake::lockFile.

src/libexpr/flake/flake.cc

@@ -298,6 +298,9 @@ LockedFlake lockFlake(
 
     auto flake = getFlake(state, topRef, lockFlags.useRegistries, flakeCache);
 
+    flake.config.apply();
+    // FIXME: send new config to the daemon.
+
     try {
 
         // FIXME: symlink attack