Filter out github-pull-request events don't run builds on irrelevant ones

[lotas]

I noticed that mozilla/task-boot is not filtering pull request events, which results in builds for such events like:

• pull_request.edited
• pull_request.ready_for_review
• pull_request.converted_to_draft
• pull_request.review_requested
• pull_request.auto_merge_enabled

We only want opened, reopened and synchronize events

Here's what it looks like in debug tool: existing .taskcluster.yml and here's how for updated .taskcluster.yml

[lotas]

Cool, thanks!