Unset restrictive umask before creating files
If a user has a restrictive umask (for example 0077) set, it will be
inherited to sudo and thus files created with sudo where not readable by
the normal user. In that case, when setting up a new chroot, the
etc/apk/repositories file would have umask 600 and a os.path.exists()
(in update_repository_list) would return false. Setting the umask to 0
first, results in world readable files and directories, so the user
running ./pmbootstrap.py install
can read them.