Rework `demo/run` so that all building happens in the runner.
Rework `demo/run` so that all building happens in the runner.

It used to pass `--build` to `samex` when running the demo. This still
works (in general) but is now avoided, so that all the build spew gets
collated before demo output.

danfuzz/samizdat,danfuzz/samizdat,danfuzz/samizdat,danfuzz/samizdat