The install phase of 'clan machines install' builds the system closure
on the target VM, which has no network access. Building kmod, unzip,
dbus (makeDBusConf), and the libxml2/libxslt doc toolchain requires all
outputs of these packages plus their sources — none of which appear in
a runtime closure. Seed every output explicitly so the remote build can
complete offline.
Fixes the nixos-test-installation{,-age,-password-store} failures in the
nixpkgs bump PR #7943.