We now make it happen later in the boot process so that multi-user
has already activated, so as to not run afoul of the logic in
switch-to-configuration.pl. It's not my favorite solution, but at
least it works. Also added a check to the VM test to catch the failure
so we don't break in future.
Fixes #23121
The initialization code is now a systemd service that explicitly
waits for network-online, so the occasional failure I was seeing
because the `nixos-rebuild` couldn't get anything from the binary
cache should stop. I hope!
