builder: keep Windows default when daemon is unavailable

[VrtxOmega]

- What I did

Kept the Windows/WCOW default builder behavior when a Windows Docker CLI client cannot read daemon OSType because the daemon is unavailable.

Fixes #6855

- How I did it

• moved the default BuildKit decision behind a helper that can distinguish daemon OSType from client OS fallback
• treats unknown daemon OSType on a Windows client like the Windows/WCOW default instead of assuming a Linux daemon
• updated builder forwarding to use BuildKitEnabled() for the default decision
• kept explicit DOCKER_BUILDKIT=1, advertised BuildKit, and Linux daemon-unavailable behavior unchanged
• added portable helper tests plus a Windows-only regression for the unavailable-daemon path

- How to verify it

GO111MODULE=auto GOPATH=<temp-gopath> go test github.com/docker/cli/cli/command github.com/docker/cli/cmd/docker -count=1
git diff --check

- Human readable description for the release notes

Avoid incorrectly warning Windows users to install Buildx when `docker build` cannot read daemon OSType.