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.