darwin-install: fix break from bad vimrc
darwin-install: fix break from bad vimrc

It looks like the `--noplugin` flag added in #5489 wasn't enough to
skirt this class of vim-init error, so this is swing 2 at a full fix.
Fixes #6462.

NixOS/nix,NixOS/nix,NixOS/nix,NixOS/nix,NixOS/nix