Add platform prefix to MultiWriter

[yun-wang]

This PR adds a platform prefix to Buildkit progress output, because the output is confusing when Newt solves multi-platform builds in parallel. However, the current implementation doesn't cover all output in plain mode. Buildkit doesn't give us a way to inject prefixes to the rest of the progress output:

• https://github.com/moby/buildkit/blob/master/util/progress/progressui/printer.go#L131
• https://github.com/moby/buildkit/blob/master/util/progress/progressui/printer.go#L198

Progress output with the current implementation:

Let me know what you think, or if you have any suggestions.

[hinshun]

I think the plain log output only prefixing the "header" is fine. They are always outputted in chunks together with the "header" so from a readability standpoint it looks good.