Update build.cake (#743)

Use release mode for script generation, otherwise sql is not generated.

Author: carlsixsmith-moj

build.cake

@@ -53,6 +53,7 @@ Task("Build")
              Arguments = "ef migrations script --no-build " +
                          "--project ./src/Infrastructure/Infrastructure.csproj " +
                          "--startup-project ./src/Server.UI/Server.UI.csproj " +
+                         "--configuration Release " +
                          "--output ./artifacts/migrations.sql --idempotent",
              WorkingDirectory = Context.Environment.WorkingDirectory // force repo root
          };
@@ -63,4 +64,4 @@ Task("Build")
  Task("Default")
     .IsDependentOn("ScriptMigration");
 
- RunTarget(target);
+ RunTarget(target);