Build stable and experimental with same command (#20573)

The goal is to simplify our CI pipeline so that all configurations
are built and tested in a single workflow.

As a first step, this adds a new build script entry point that builds
both the experimental and stable release channels into a single
artifacts directory.

The script works by wrapping the existing build script (which only
builds a single release channel at a time), then post-processing the
results to match the desired filesystem layout. A future version of the
build script would output the files directly without post-processing.

Because many parts of our infra depend on the existing layout of the
build artifacts directory, I have left the old workflows untouched.
We can incremental migrate to the new layout, then delete the old
workflows after we've finished.