darwin-rebuild: add -H to sudo for $systemConfig/activate

[zowoq]

#509 (comment)
It might be nice to add -H to the sudo "$systemConfig/activate" line as well. A quick skim of the activate script doesn't show any references to $HOME but it's still probably a good idea for consistency, especially as someone could write a custom activation script that references this variable or invokes a command that itself consults this variable, and system activation should probably try to avoid any dependency on the initiating user.

Running nix store diff-closure in a activation script displays:

warning: $HOME ... is not owned by you, falling back to the one defined in the 'passwd' file.

[Enzime]

LGTM