Auto merge of #29601 - servo:debug-wpt-update-script, r=delan

Fix the WPT export script

- Have the WPT exporter script use the WPT_SYNC_TOKEN for checking out
  wpt.
- Make sure the local WPT repository is unshallow when pushing.
- When searching for existing PRs use the main GitHub search API,
  as the pull request search does not seem to properly process
  the "head" parameter.
- When deleting branches in the downstream WPT repository, use
  the full URL to avoid trying to modify the upstream repository.

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- [x] `./mach test-tidy` does not report any errors
- [x] There are tests for these changes OR

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