refactor: Use new GetConfigFilePath function

New function was introduced by willcl-ark <will@256k1.dev> in commit
56e370f from
bitcoin/bitcoin#27073 and removes some duplicate code.

Author: ryanofsky

src/bitcoin-cli.cpp

@@ -817,7 +817,7 @@ static UniValue CallRPC(BaseRequestHandler* rh, const std::string& strMethod, co
         if (failedToGetAuthCookie) {
             throw std::runtime_error(strprintf(
                 "Could not locate RPC credentials. No authentication cookie could be found, and RPC password is not set.  See -rpcpassword and -stdinrpcpass.  Configuration file: (%s)",
-                fs::PathToString(GetConfigFile(gArgs.GetPathArg("-conf", BITCOIN_CONF_FILENAME)))));
+                fs::PathToString(gArgs.GetConfigFilePath())));
         } else {
             throw std::runtime_error("Authorization failed: Incorrect rpcuser or rpcpassword");
         }

src/init/common.cpp

@@ -121,7 +121,7 @@ bool StartLogging(const ArgsManager& args)
     LogPrintf("Using data directory %s\n", fs::PathToString(gArgs.GetDataDirNet()));
 
     // Only log conf file usage message if conf file actually exists.
-    fs::path config_file_path = GetConfigFile(args.GetPathArg("-conf", BITCOIN_CONF_FILENAME));
+    fs::path config_file_path = args.GetConfigFilePath();
     if (fs::exists(config_file_path)) {
         LogPrintf("Config file: %s\n", fs::PathToString(config_file_path));
     } else if (args.IsArgSet("-conf")) {

src/qt/guiutil.cpp

@@ -428,7 +428,7 @@ void openDebugLogfile()
 
 bool openBitcoinConf()
 {
-    fs::path pathConfig = GetConfigFile(gArgs.GetPathArg("-conf", BITCOIN_CONF_FILENAME));
+    fs::path pathConfig = gArgs.GetConfigFilePath();
 
     /* Create the file */
     std::ofstream configFile{pathConfig, std::ios_base::app};