ci: maintainer_check: Use zephyrbot token

This commit updates the maintainer check workflow to use the zephyrbot "PR
assigner" token with `org:read` permission because the default workflow
token is not able to retrieve organisation-level user membership
information.

Note that the workflow trigger condition is changed from `pull_request` to
`pull_request_target` because non-workflow secrets may not be accessed from
`pull_request` trigger for security reasons.

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