Support multiple --cache-from sources

[schaffe]

Hello, please add support for multiple cache-from sources. Currently it is possible with Docker CLI:

> docker build --help
Start a build

Usage:  docker buildx build [OPTIONS] PATH | URL | -
...
      --cache-from stringArray        External cache sources (e.g., "user/app:cache", "type=local,src=path/to/dir")

The use case that we have involves multiple cache sources and missing it in Kaniko is a significant hit for performance of our builds.