fix: ignore unknown/unknown platform in cmd/combine

[imjasonh]

To avoid a similar future issue to tektoncd/pipeline#6260, ignore unknown/unknown which buildkit might start injecting into index manifests in the future.

/kind bug

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• unit tests failed immediately, but the link goes to a Prow page that says the test has been running for 36 minutes with no link to logs that I can find. /test also didn't rerun the check.

[imjasonh]

/assign @vdemeester

[imjasonh]

/test check-pr-has-kind-label

[tekton-robot]

@imjasonh: The specified target(s) for /test were not found.
The following commands are available to trigger required jobs:

• /test pull-dogfooding-mario-test
• /test pull-tekton-plumbing-check-testgrid-config

In response to this:

/test check-pr-has-kind-label

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[imjasonh]

/kind bug

[imjasonh]

/test plumbing-unit-tests

[tekton-robot]

@imjasonh: The specified target(s) for /test were not found.
The following commands are available to trigger required jobs:

• /test pull-dogfooding-mario-test
• /test pull-tekton-plumbing-check-testgrid-config

In response to this:

/test plumbing-unit-tests

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.

[wlynch]

/lgtm

[tekton-robot]

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