Pass all git-related auth via supported buildkit constructs

[vladaionescu]

Currently git auth is passed automatically to the buildkit daemon via some mounts and env vars. This requires meddling with the buildkit daemon in a manner that is difficult to translate to a build-in-the-cloud setup. A better approach is to pass in this auth via the client-daemon connection:

• For username-password, use buildkit's newly supported token-based auth
• For ssh-based auth, buildkit requires a contribution

[mxey]

For ssh-based auth, buildkit requires moby/buildkit#1564

This has apparently been implemented.