arch-nspawn: Use a unique scope name instead of --keep-unit

--keep-unit is really only for use in services like systemd-nspawn@.service.

The parameter was added in commit 000ea6c7 because systemd-nspawn defaults the name of the machine (and thus the generated scope) to the name of the working directory, which is not unique. Thus spawning a container from archbuild/extra-x86_64/foo while archbuild/testing-x86_64/foo is already running would fail.

We can avoid the unit conflict by giving the container a unique machine name. Creating a scope also allows us to place the container in a slice hierarchy for resource control.