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 000ea6c7bb 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.

src/arch-nspawn.in

@@ -60,7 +60,8 @@ nspawn_args=(
 	--directory="$working_dir"
 	--setenv="PATH=/usr/local/sbin:/usr/local/bin:/usr/bin"
 	--register=no
-	--keep-unit
+	--slice="devtools-$(systemd-escape "${SUDO_USER:-$USER}")"
+	--machine="arch-nspawn-$$"
 	--as-pid2
 )