BuildKit: Malicious client can bypass destination directory validation on local sources upload

BuildKit is a toolkit for converting source code to build artifacts in an efficient, expressive and repeatable manner. Prior to 0.31.2, a custom client can produce such an upload request to the BuildKit daemon that files can escape from the BuildKit-controlled state directory. The client needs to have valid permissions to access BuildKit control API to issue builds, eg., bypass authentication, etc. This issue is fixed in version 0.31.2.