ci: build a second docker image with ffmpeg build context

[iameli]

Looks like https://github.com/livepeer/go-livepeer/pull/2499 broke some downstream projects by moving to buildx in go-livepeer; orch-tester and task-runner both depend on livepeerci/build for their ffmpeg build contexts so that they can embed go-livepeer as a library. Previously livepeerci/build was a separate Docker image containing the build stage of the go-livepeer build; now it contains the buildx cache.

So... it'd be nice to do a separate build with --target=build of the same Dockerfile and upload it so those projects can use it. Perhaps livepeerci/ffmpeg-build-context?