`darwin-rebuild` broken on 10.14

[quentinmit]

It looks like #799 broke darwin-rebuild on 10.14 because sudo doesn't recognize --preserve-env=PATH:

$ darwin-rebuild switch -L --flake . --show-trace
building the system configuration...
sudo: option `--preserve-env' doesn't allow an argument
usage: sudo -h | -K | -k | -V
usage: sudo -v [-AknS] [-g group] [-h host] [-p prompt] [-u user]
usage: sudo -l [-AknS] [-g group] [-h host] [-p prompt] [-U user] [-u user] [command]
usage: sudo [-AbEHknPS] [-C num] [-g group] [-h host] [-p prompt] [-u user] [VAR=value] [-i|-s] [<command>]
usage: sudo -e [-AknS] [-C num] [-g group] [-h host] [-p prompt] [-u user] file ...

That arg probably needs to be conditionalized on something (sudo version? Darwin version?)

[Enzime]

Fixed by #831