FIX: Skip rebaking posts with inline GitHub PR oneboxes (#36576)

Posts and chat messages with inline oneboxes to GitHub PRs were being
unnecessarily rebaked when the PR status changed. This happened because
the check `cooked.include?("onebox")` matched both full oneboxes
(`class="onebox githubpullrequest"`) and inline oneboxes
(`class="inline-onebox"`).

Changed the check to match "githubpullrequest" specifically, which only
appears in full PR oneboxes that actually display the status.

Internal ref - t/169442

plugins/discourse-github/app/jobs/regular/rebake_github_pr_posts.rb

@@ -46,7 +46,7 @@ module Jobs
 
     def has_github_pr_onebox?(cooked, pr_url)
       # quick & dirty check to avoid doing unnecessary rebakes
-      cooked.present? && cooked.include?("onebox") && cooked.include?(pr_url)
+      cooked.present? && cooked.include?("githubpullrequest") && cooked.include?(pr_url)
     end
   end
 end