Set `BUILDX_BUILDER` environment variable

[haines]

I was trying to work out why one of our build scripts wasn't using the Blacksmith builder - it turns out we were running docker build rather than docker buildx build.

https://docs.docker.com/build/builders/#difference-between-docker-build-and-docker-buildx-build

The docker build command always defaults to using the default builder that comes bundled with the Docker Engine, to ensure backwards compatibility with older versions of the Docker CLI. The docker buildx build command, on the other hand, checks whether you've set a different builder as the default builder before it sends your build to BuildKit.

It would be helpful if this action set the BUILDX_BUILDER environment variable for subsequent job steps so that plain docker build commands would use the right builder instance.