--platform can be ignored by what the base image supports

[benmoss]

If you have a defaultBaseImage that is an image manifest rather than a image index, it seems that ko will just build for that image's platform, happily ignoring whatever you pass via --platforms.

Repro here: https://github.com/benmoss/wtf-ko

❯ ko build --platform=all .
2022/05/05 10:57:43 Using base gcr.io/distroless/static@sha256:abb120b4ebb4e734dc4c82181bb32b5a4f8bcf78e4540b094b9d59ba7a29a5ad for github.com/benmoss/wtf-ko
2022/05/05 10:57:43 Building github.com/benmoss/wtf-ko for linux/amd64
2022/05/05 10:57:44 Loading kind.local/wtf-ko-3d3e10e27ff19c6d1d54aaa50fb62fd9:843abadc2e51b54f34fe7c0bee50f99c4462593efc183b6867b72573aa0bf572
2022/05/05 10:57:44 Loaded kind.local/wtf-ko-3d3e10e27ff19c6d1d54aaa50fb62fd9:843abadc2e51b54f34fe7c0bee50f99c4462593efc183b6867b72573aa0bf572
2022/05/05 10:57:44 Adding tag latest
2022/05/05 10:57:44 Added tag latest
kind.local/wtf-ko-3d3e10e27ff19c6d1d54aaa50fb62fd9:843abadc2e51b54f34fe7c0bee50f99c4462593efc183b6867b72573aa0bf572

[benmoss]

--platform=all is actually kind of ambiguous, since I think it tends to mean "build all platforms supported by the base", so this might technically be correct.

This might be a better demonstration of the problem:

❯ ko build --platform=linux/arm64 .
2022/05/05 11:10:52 Using base gcr.io/distroless/static@sha256:abb120b4ebb4e734dc4c82181bb32b5a4f8bcf78e4540b094b9d59ba7a29a5ad for github.com/benmoss/wtf-ko
2022/05/05 11:10:52 Building github.com/benmoss/wtf-ko for linux/amd64
2022/05/05 11:10:52 Loading kind.local/wtf-ko-3d3e10e27ff19c6d1d54aaa50fb62fd9:843abadc2e51b54f34fe7c0bee50f99c4462593efc183b6867b72573aa0bf572
2022/05/05 11:10:53 Loaded kind.local/wtf-ko-3d3e10e27ff19c6d1d54aaa50fb62fd9:843abadc2e51b54f34fe7c0bee50f99c4462593efc183b6867b72573aa0bf572
2022/05/05 11:10:53 Adding tag latest
2022/05/05 11:10:53 Added tag latest
kind.local/wtf-ko-3d3e10e27ff19c6d1d54aaa50fb62fd9:843abadc2e51b54f34fe7c0bee50f99c4462593efc183b6867b72573aa0bf572

[github-actions]

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