eks-prow-build-cluster: fix port number in Grafana's config

[pkprzekwas]

This PR makes sure of using the right port in EKS prow build cluster's Grafana instance configuration.

Part of #4686
/assign @ameukam @xmudrii

[k8s-ci-robot]

Hi @pkprzekwas. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[xmudrii]

/lgtm
/assign @ameukam
for approval

[xmudrii]

/ok-to-test

[ameukam]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ameukam, pkprzekwas, xmudrii

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