util: Do not use gArgs global in ArgsManager member functions

Summary: This is a backport of [[bitcoin/bitcoin#20092 | core#20092]]

Test Plan: `ninja all check-all`

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10429

src/util/system.cpp

@@ -587,7 +587,7 @@ void ArgsManager::ClearForcedArg(const std::string &strArg) {
 }
 
 std::string ArgsManager::GetHelpMessage() const {
-    const bool show_debug = gArgs.GetBoolArg("-help-debug", false);
+    const bool show_debug = GetBoolArg("-help-debug", false);
 
     std::string usage = "";
     LOCK(cs_args);
@@ -1015,7 +1015,7 @@ bool ArgsManager::ReadConfigFiles(std::string &error,
     ClearDatadirCache();
     if (!CheckDataDirOption()) {
         error = strprintf("specified data directory \"%s\" does not exist.",
-                          gArgs.GetArg("-datadir", "").c_str());
+                          GetArg("-datadir", "").c_str());
         return false;
     }
     return true;