The `matrix-client-commet` role's pull task references
`matrix_client_commet_container_image_force_pull`, but the variable
was never defined in defaults. Users setting
`matrix_client_commet_container_image_self_build: false` would hit an
undefined-variable failure.

Define it using the same pattern as other client roles (force-pull on
moving tags), covering both `:latest` and `:main` since Commet's
default version is `main`.