abuild: filter out security.selinux xattrs

This modifies abuild's behaviour to be independent of whether SELinux labels are present in the build environment.

This is a workaround for containerised build environments where the hosting environment has SELinux enabled. Security labels leak into the container, manifesting as xattrs.

Discussion thread: https://lists.alpinelinux.org/~alpine/devel/%3CCA%2Bgy4ieuXLuQddxmPuiucyZbut%3D14cR8tgmGrh0qE9qLDBivmg%40mail.gmail.com%3E