abuild: set POSIXLY_CORRECT=1 when using patch

gnu patch creates .orig files when the patch didn't match exactly. This
can cause additional files in the build output or even build failures.

When POSIXLY_CORRECT is set gnu patch uses the posix behaviour which is
to not create .orig files.

Ref https://pkgs.alpinelinux.org/contents?file=*.orig&arch=x86_64