feat(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 000ea6c7bbf6191b 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.

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