nixos: run parted with --script option

-s, --script: never prompts for user intervention

Sometimes the NixOS installer tests fail when they invoke parted, e.g.
https://hydra.nixos.org/build/62513826/nixlog/1. But instead of exiting
right there, the tests hang until the Nix builder times out (and kills
the build). With this change the tests would instead fail immediately,
which is preferred.

While at it, use "parted --script" treewide, so nobody gets build
timeout due to parted error (or misuse). (Only nixos/ use it, and only
non-interactive.)

A few instances already use the short option "-s", convert them to long
option "--short".