Ensure log path exists

Nothing else creates the directory, and on mac the log is not in the
default user cache path like on Windows, which means we can't rely
on the settings class to create the log path

Author: shana

src/GitHub.Api/Platform/DefaultEnvironment.cs

@@ -46,6 +46,7 @@ public DefaultEnvironment()
             {
                 LogPath = UserCachePath.Combine(logFile);
             }
+            LogPath.EnsureParentDirectoryExists();
         }
 
         public DefaultEnvironment(ICacheContainer cacheContainer) : this()