%BASE% was broken

I haven't pinned down which change caused this, but `%BASE%` was supposed to point to a directory and not the executable.

ServiceDescriptor.cs

@@ -67,7 +67,7 @@ namespace winsw
             dom.Load(BasePath + ".xml");
 
             // register the base directory as environment variable so that future expansions can refer to this.
-            Environment.SetEnvironmentVariable("BASE", p);
+            Environment.SetEnvironmentVariable("BASE", d.FullName);
             // ditto for ID
             Environment.SetEnvironmentVariable("SERVICE_ID", Id);
             Environment.SetEnvironmentVariable("WINSW_EXECUTABLE", ExecutablePath);