K8SPXC-1843: fix handle_sigterm in run_backup.sh

[pooknull]

https://perconadev.atlassian.net/browse/K8SPXC-1843

DESCRIPTION

Problem:
If the joiner/garbd and donor disconnect from each other, the backup can remain stuck in the Running state instead of failing.

Cause:
run_backup.sh did not handle SIGTERM from the interrupted SST. In the this scenario, the script could stay in an incomplete state instead of exiting with an error, so the backup job never failed.

Solution:
Replace if ((FIRST_RECEIVED == 0)) check, which previously made the backup job hanging, with exit 1

CHECKLIST

Jira

• Is the Jira ticket created and referenced properly?
• Does the Jira ticket have the proper statuses for documentation (Needs Doc) and QA (Needs QA)?
• Does the Jira ticket link to the proper milestone (Fix Version field)?

Tests

• Is an E2E test/test case added for the new feature/change?
• Are unit tests added where appropriate?
• Are OpenShift compare files changed for E2E tests (compare/*-oc.yml)?

Config/Logging/Testability

• Are all needed new/changed options added to default YAML files?
• Are all needed new/changed options added to the Helm Chart?
• Did we add proper logging messages for operator actions?
• Did we ensure compatibility with the previous version or cluster upgrade process?
• Does the change support oldest and newest supported PXC version?
• Does the change support oldest and newest supported Kubernetes version?