ci: assigner: Use zephyrbot token

This commit updates the pull request assigner workflow to use the zephyrbot
token instead of the workflow token.

Note that the workflow token, by design, cannot access the
organisation-level user membership information, and may cause the assigner
script to return `Skip 'user': not in collaborators` even for valid
collaborators.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>