fix mtu issue on ppc64le build-tool.

[morvencao]

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[ibm-ci-bot]

@morvencao: Updated the job-config configmap in namespace default using the following files:

• key IBM.test-infra.yaml using file prow/cluster/jobs/IBM/test-infra/IBM.test-infra.yaml

In response to this:

What this PR does / why we need it:

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 #

Specify your PR reviewers:

/cc @gyliu513 @morvencao

Special notes for your reviewer:

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Release note:

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.