AllowListInputAccessor has the invariant that if a path is accessible,
its parent directories are also considered accessible (though reading
them only yields the allowed subdirectories). As a result
`builtins.pathExists "/nix/store"` returns true.

However this wasn't the behaviour of previous path access control,
where `builtins.pathExists "/nix/store"` returns false even if a
subdirectory of the store is accessible.

Fixes NixOS#9672.

'--restrict-eval true' was probably not what was intended
(i.e. parsing a file named 'true').

E.g. `pathExists` on such parents will return `false`, and `readDir`
will fail.