add in-place mode for installation on hosts with small RAM #1138
Description
I found myself in a position to provision a vhost that can't physically fit its entire store in RAM. It was not practical to push the pre-build image to that host. The host only has one disk, which is the install disk.
Under these circumstances, I see no other way but to use the target disk in-place for the fetching of all packages (instead of pref-etching to the install images tmpfs backed /nix/store).