Revert "Use posix_spawn to run the pager"

This reverts commit d34d2b2bbf784c0bb420a50905af25e02c6e4989.
Don't wait for PID -1

The pid field can be -1 if forking the substituter process failed.
Include cacert in the binary tarball

This prevents having to fetch Nixpkgs or cacert over http.