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/rpc/request.cpp

@@ -75,7 +75,7 @@ static fs::path GetAuthCookieFile(bool temp=false)
     if (temp) {
         arg += ".tmp";
     }
-    return AbsPathForConfigVal(arg);
+    return AbsPathForConfigVal(gArgs, arg);
 }
 
 bool GenerateAuthCookie(std::string *cookie_out)