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/bitcoind.cpp

@@ -150,7 +150,7 @@ static bool AppInit(NodeContext& node, int argc, char* argv[])
     std::any context{&node};
     try
     {
-        if (!CheckDataDirOption()) {
+        if (!CheckDataDirOption(args)) {
             return InitError(Untranslated(strprintf("Specified data directory \"%s\" does not exist.\n", args.GetArg("-datadir", ""))));
         }
         if (!args.ReadConfigFiles(error, true)) {