The default GITHUB_TOKEN is created for 'github-bot' user and has limitations.
Specifically, events generated by this github-bot
cannot trigger additional workflows.
This PR uses fine-grained PAT generated for @servo-bot account with the
permissions scoped to servo/servo repo and grants the
'contents: write' and 'pull_request: write' permissions.
Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>