Re-enable systemd-nspawn test

It was disabled in c6953d1ff6 because
a recent Nixpkgs bump brought in a new systemd which changed how
systemd-nspawn worked.

As far as I can tell, the issue was caused by this upstream systemd
commit:
b71a0192c0

Bind-mounting the host's `/sys` and `/proc` into the container's
`/run/host/{sys,proc}` fixes the issue and allows the test to succeed.

tests/nixos/containers/systemd-nspawn.nix

@@ -73,6 +73,8 @@ runCommand "test"
       --resolv-conf=off \
       --bind-ro=/nix/store \
       --bind=$out \
+      --bind=/proc:/run/host/proc \
+      --bind=/sys:/run/host/sys \
       --private-network \
       $toplevel/init
   ''