buildkit: Specify build platforms based on that of the workers

Without an explicitly provided build platform, dockerfile2llb seems to infer one from the target platforms. This is not desirable behavior when the platform doesn't match that of the worker, as it will have to be emulated which may not even be possible in all build environments (requires a working QEMU) or at the very least very slow.

Let's guarantee that the build platform will match that of the workers or the buildkitd host (they should be the same in our setup).

Bug: T318866