🌱 Move builds for ghcr.io/kcp-dev/infra/build to prow

[embik]

Initially I added builds for ghcr.io/kcp-dev/infra/build in #35 via GitHub Actions because that was how kcp-dev/kcp was doing it. Now the core repository has migrated to building images in prow, so this PR adapts the changes made there (kcp-dev/kcp#2976) into the infra repository for our build image as well.

I'm leaving the multi-arch aspect in because I find it useful, it's less divergence from the kcp script and it could unlock running jobs on ARM if we later chose so, by just dropping that architecture in the list for our build image.

[embik]

/cc @xrstf

[xrstf]

/approve
/lgtm

[kcp-ci-bot]

LGTM label has been added.

Git tree hash: e1c808454abe6ccf71109b838354f26fe9abb796

[kcp-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xrstf

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[kcp-ci-bot]

@embik: Updated the job-config configmap in namespace prow at cluster default using the following files:

• key prow-jobs-infra-infra-postsubmits.yaml using file prow/jobs/infra/infra-postsubmits.yaml
• key prow-jobs-infra-infra-presubmits.yaml using file prow/jobs/infra/infra-presubmits.yaml

In response to this:

Initially I added builds for ghcr.io/kcp-dev/infra/build in #35 via GitHub Actions because that was how kcp-dev/kcp was doing it. Now the core repository has migrated to building images in prow, so this PR adapts the changes made there (kcp-dev/kcp#2976) into the infra repository for our build image as well.

I'm leaving the multi-arch aspect in because I find it useful, it's less divergence from the kcp script and it could unlock running jobs on ARM if we later chose so, by just dropping that architecture in the list for our build image.

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