Don't enable the NVIDIA driver by default because it's unfree
nixos-generate-config: Use stable device paths (e.g. /dev/disk/by-uuid/X)
nixos-generate-config: Don't include /var/setuid-wrappers
nixos-generator-config: Don't emit a double / in bind mounts
nixos-install: Don't pass --show-trace by default
nixos-install: Run in a private mount namespace

This ensures that all mounts are automatically cleaned up.
nixos-install: Assume the build user group is "nixbld"

The build user group is always "nixbld", so no need to detect it.
nixos-install: Don't copy the bootstrap Nix if it's already there

This makes re-running nixos-install a bit faster.
nixos-install: Ask the user to set a root password

This removes the need to have an initially empty root password.
nixos-install: Add operation --chroot

"nixos-install --chroot" runs a command (by default a login shell) in
a chroot inside the NixOS installation in /mnt. This might useful for
poking around a new installation.
nixos-install: Don't bind-mount all of /etc

We only need a copy of /etc/resolv.conf for networking, and
/etc/{passwd,group} for building.
/var/run -> /run