Make WRKSRC files copied from FILESDIR writable

The contents of FILESDIR may or or may have the user write bit set.
They contain generated files (list.h), and libarchive's build system
recreates these files by opening them for writing without first
removing them.

Discussed on tech-pkg@; thanks to dholland@ for suggesting chmod -R as
both the simplest change and the one least likely to need adjusting.

(gdt)