This reverts commit
c37e76b4d2. Unfortunately, using
"machinectl shell" has two bad side effects:
* It sends the command's stderr to stdout.
* It doesn't propagate the command's exit status.
This broke NixOps.
PR #18825.
This moves nixos-containers into its own package so that it can be
relied upon by other packages/systems. This should make development
using dynamic containers much easier.