Factor some environment setting into set_scoped_internal

Breaks up a monolith function.

src/fish.cpp

@@ -402,13 +402,12 @@ int main(int argc, char **argv) {
         save_term_foreground_process_group();
     }
 
-    auto &globals = env_stack_t::globals();
     const struct config_paths_t paths = determine_config_directory_paths(argv[0]);
     env_init(&paths);
     // Set features early in case other initialization depends on them.
     // Start with the ones set in the environment, then those set on the command line (so the
     // command line takes precedence).
-    if (auto features_var = globals.get(L"fish_features")) {
+    if (auto features_var = env_stack_t::globals().get(L"fish_features")) {
         for (const wcstring &s : features_var->as_list()) {
             mutable_fish_features().set_from_string(s);
         }