make bors ignore comments in PR template

.github/pull_request_template.md

@@ -1,3 +1,4 @@
+<!-- homu-ignore:start -->
 <!--
 If this PR is related to an unstable feature or an otherwise tracked effort,
 please link to the relevant tracking issue here. If you don't know of a related
@@ -7,4 +8,5 @@ This PR will get automatically assigned to a reviewer. In case you would like
 a specific user to review your work, you can assign it to them by using
 
     r​? <reviewer name>
--->
+-->
+<!-- homu-ignore:end -->