this removes the need for the windows version's OpenSSH.Server
feature-on-demand package which is not publicly available.
fixes gitea issue #6 <M-Labs/wfvm#6>
This is useful if the install.wim contains multiple Windows versions
(e.g., Home, Pro, ..), because the autounattend file will always select
index 1. With this mechanism, a variant different from the first one can
be automatically selected. imageSelection can be either an index (1-N)
or the image name wiminfo can list all images contained in a given WIM
file.
The default case is index 1, which has the same effect as before, with
the possibility of having a slightly smaller install.wim file because
all unwanted variants are discarded.
With newer Windows 10 versions, `install.wim` can become larger than
4GiB, which can't be placed in a FAT32 partition anymore. By splitting
it into chunks with `wimsplit` and removing `install.wim`, the larger
images work fine.
Since we set RTC in the Nix build it's likely that any passwords have
expired since the image generation RTC time.
Ideally this should be applied in autounattend.xml but it didn't work
out either as FirstLogonCommands or as a command in the specialize pass.
