Add a default BuildConstants and change encoding on save (#1026)

src/Runner.Sdk/BuildConstants.cs

@@ -0,0 +1,19 @@
+namespace GitHub.Runner.Sdk
+{
+    /***
+     * WARNING: This file is automatically regenerated on layout so the runner can provide version/commit info (do not manually edit it).
+     */
+    public static class BuildConstants
+    {
+        public static class Source
+        {
+            public static readonly string CommitHash = "N/A";
+        }
+
+        public static class RunnerPackage
+        {
+            public static readonly string PackageName = "N/A";
+            public static readonly string Version = "0";
+        }
+    }
+}