Moved initialization from Start() to Awake() (googleforgames#1338)

Awake() is more commonly used to initialize variables as it is
guaranteed to run as soon as the game object is instantiated or
activated. Start() is called before the first Update(), which
might be up to a frame later.

See Unity docs for reference
https://docs.unity3d.com/ScriptReference/MonoBehaviour.Awake.html

sdks/unity/AgonesSdk.cs

@@ -59,10 +59,14 @@ private struct KeyValueMessage
 
         #region Unity Methods
         // Use this for initialization.
-        private void Start()
+        private void Awake()
         {
             String port = Environment.GetEnvironmentVariable("AGONES_SDK_HTTP_PORT");
             sidecarAddress = "http://localhost:" + (port ?? "9358");
+        }
+
+        private void Start()
+        {
             HealthCheckAsync();
         }