patch(1) has a -d/--directory parameter which allows us to specify a directory
where the patch will be applied. This is especially useful when we have
multiple distfiles and want to patch in $build_wrksrc because that's where the
patch is usually applied.
Problem is, that the -i parameter is also relative to the the -d parameter and
thus fails to find the patch, when -d is set. We solve that by using standard
input instead of -i.
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