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.

.github/workflows/upstream-wpt-changes.yml

@@ -25,7 +25,12 @@ jobs:
         with:
           path: wpt
           repository: 'web-platform-tests/wpt'
-          token: ${{ secrets.GITHUB_TOKEN }}
+          # The token here must be the token that we will use to push to the
+          # WPT repository and not the token used for GitHub actions, because
+          # the checkout action sets up an `extraheader` authorization override
+          # using the token specified here.
+          # See https://github.com/actions/checkout/issues/162.
+          token: ${{ secrets.WPT_SYNC_TOKEN }}
       - name: Install requirements
         run: pip install -r servo/etc/ci/upstream-wpt-changes/requirements.txt
       - name: Process pull request