Fix installation of versions on ubuntu

This fixes the installation of versions under Ubuntu as mention in #14 and #15. The problem is that the `&>` is interpreted wrong and forces the operation to go to the background and therefore is the current working dir not the expected `$N_PREFIX/n`. Instead the `cwd` is the directory from where the command was executed.

Also it is executing the dependent commands before the download and unpacking is finished, which leads to further problems.