linuxkit: fix pkg build on Windows
linuxkit: fix pkg build on Windows

Previously when we set `cmd.Stderr = os.Stderr`, the stderr from buildx
would be mixed with the image tar, corrupting it.

Work around this (Windows-specific) problem by adding an explicit
indirection via a io.Pipe()

Signed-off-by: David Scott <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@recoil.org>

djs55/linuxkit,rn/linuxkit,linuxkit/linuxkit,rn/linuxkit,furious-luke/linuxkit,JohnnyLeone/linuxkit,deitch/linuxkit,linuxkit/linuxkit,djs55/linuxkit,deitch/linuxkit,furious-luke/linuxkit,djs55/linuxkit,linuxkit/linuxkit,furious-luke/linuxkit,linuxkit/linuxkit,furious-luke/linuxkit,rn/linuxkit,JohnnyLeone/linuxkit,JohnnyLeone/linuxkit,deitch/linuxkit,JohnnyLeone/linuxkit,linuxkit/linuxkit,djs55/linuxkit,rn/linuxkit,deitch/linuxkit,furious-luke/linuxkit,rn/linuxkit,djs55/linuxkit,deitch/linuxkit,JohnnyLeone/linuxkit