abuild: filter out security.selinux xattrs

This patch 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
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