nixos-install: error out if $mountPoint has bad permissions

The nix store more-or-less requires o+rx on all parent directories.
This is primarily because nix runs builders in a uid/gid mapped
user-namespace, and those builders have to be able to operate on the nix
store.

This check is especially helpful because nix does not produce a helpful
error on its own (rather, creating directories and such works, it's not
until 'mount --bind' that it gets an EACCES).

Helps users who run into this opaque error, such as in #67465.
Possibly fixes that issue if bad permissions were the only cause.