Fix string interpolation for path in log (#2630)

## Why make this change?

fixes #2629 

## What is this change?

Fix the string interpolation

Author: simonsabin

src/Config/FileSystemRuntimeConfigLoader.cs

@@ -273,13 +273,14 @@ public bool TryLoadConfig(
             return false;
         }
 
-        string errorMessage = "Unable to find config file: {path} does not exist.";
         if (logger is null)
         {
+            string errorMessage = $"Unable to find config file: {path} does not exist.";
             Console.Error.WriteLine(errorMessage);
         }
         else
         {
+            string errorMessage = "Unable to find config file: {path} does not exist.";
             logger.LogError(message: errorMessage, path);
         }