gstlal_ll_inspiral_event_uploader: rename duplicated heartbeat variable

Rename accidentally duplicated heartbeat variable in gstlal_ll_inspiral_event_uploader. The job heartbeats defined in the EventProcessor class use this variable but the same variable was used for the inspiral heartbeats (used to monitor status of Kafka, ie if the job is successfully consuming messages).

Reviewed, see: https://git.ligo.org/gstlal/o4-code-review/-/issues/28