ci: manifest: Use 'zephyrbot' token to run manifest action

This commit updates the CI manifest workflow to run the manifest action
as the 'zephyrbot' user instead of the 'github-actions' user.

The 'github-actions' user does not have the permissions required to
trigger another workflow and fails to trigger the "Do Not Merge"
workflow when it (un)labels a pull request with the "DNM" label.

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