refactor: Stop using gArgs global in system.cpp

Most of the code in util/system.cpp that was hardcoded to use the global
ArgsManager instance `gArgs` has been changed to work with explicit ArgsManager
instances (for example in https://github.com/bitcoin/bitcoin/pull/20092). But a
few hardcoded references to `gArgs` remain. This commit removes the last ones
so these functions aren't reading or writing global state.

src/qt/bitcoin.cpp

@@ -588,7 +588,7 @@ int GuiMain(int argc, char* argv[])
     if (!Intro::showIfNeeded(did_show_intro, prune_MiB)) return EXIT_SUCCESS;
 
     /// 6a. Determine availability of data directory
-    if (!CheckDataDirOption()) {
+    if (!CheckDataDirOption(gArgs)) {
         InitError(strprintf(Untranslated("Specified data directory \"%s\" does not exist.\n"), gArgs.GetArg("-datadir", "")));
         QMessageBox::critical(nullptr, PACKAGE_NAME,
             QObject::tr("Error: Specified data directory \"%1\" does not exist.").arg(QString::fromStdString(gArgs.GetArg("-datadir", ""))));