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/bitcoin-cli.cpp

@@ -161,7 +161,7 @@ static int AppInitRPC(int argc, char* argv[])
         }
         return EXIT_SUCCESS;
     }
-    if (!CheckDataDirOption()) {
+    if (!CheckDataDirOption(gArgs)) {
         tfm::format(std::cerr, "Error: Specified data directory \"%s\" does not exist.\n", gArgs.GetArg("-datadir", ""));
         return EXIT_FAILURE;
     }