CI: Enable parallel mode for the coveralls

[tenzen-y]

What this PR does / why we need it:
We sometimes face the following error when we retry CI. So, I enabled the parallel mode to avoid the error.

bad response status from coveralls: 422
{"message":"Can't add a job to a build that is already closed. Build 8475719809 is closed. See docs.coveralls.io/parallel-builds","error":true}
Error: Error: The process '/home/runner/work/_actions/shogo82148/actions-goveralls/v1/bin/goveralls_linux_amd64' failed with exit code 1

https://github.com/kubeflow/katib/actions/runs/8475719809/job/23237738755?pr=2285#step:5:29

Docs: https://github.com/shogo82148/actions-goveralls?tab=readme-ov-file#parallel-job-example

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Checklist:

• Docs included if any changes are user facing

[andreyvelich]

Thank you @tenzen-y !
/lgtm

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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