Buildx def

[crazy-max]

First spike to default buildx on docker/cli.

Don't take into account the GitHub Actions cross commit 31fe2f1. Just here to be able to produce temp artifacts in the workflow for testing.

cc @tonistiigi @thaJeztah

[crazy-max]

Message if buildx is missing or broken:

$ docker build .
failed to fetch metadata: fork/exec /usr/libexec/docker/cli-plugins/docker-buildx: no such file or directory

ERROR: Missing or broken buildx plugin. To install buildx, see
       https://docs.docker.com/buildx/working-with-buildx/#install. You can
       also fallback to the legacy builder with: DOCKER_BUILDKIT=0

[crazy-max]

Waiting for docker#3234 before creating a draft PR on docker/cli.

[tonistiigi]

Message if buildx is missing or broken:

We could do a better error without fork/exec but checking if the file exists.

Other error is expected. Maybe wording should be "Your docker installation is missing builder component buildx"

Fallback should be opt-in only. If there is no buildx then showing this message by default is correct.