The code in this repository illustrates how to convert a flake that uses nixosConfigurations from flake-utils to flake-parts.
The conclusions so far:
nixosConfigurations is supposed to be hermetic, i.e. it should not take any
information about the build or the host system from the outside.
Not taking the build system information is disappointing but could be justified by the fact that building on different systems is not likely to produce exactly the same output.
One way to deal with it is to designate one system as official. Users of other
systems have to use --impure so their systems can be detected.
It's possible to remove nixosConfigurations from the flake completely if that
limitation is unacceptable.
Another option is to have separate configurations for different build system. Refactoring of the resulting code is possible, but it looks quite complex.