Installer should reset umask: /nix ends up unreadable w/ unusual umask

I installed Determinate Nix using the normal flow and got unusual permissions on `/nix` and `/etc/nix`: ``` $ ls -ld /nix drwx------ 1 root root 66 Mar 20 14:37 /nix ``` ``` $ ls -l /etc/nix/nix.conf -rw------- 1 root root 1262 Mar 20 14:37 /etc/nix/nix.conf ``` I suspect it's because I use a tighter-than-usual umask, `0077`. The installer probably wants to reset the umask during the installation.