…inking

Symlinking works for most plugins and themes, but Avada, for instance, fails to
understand the symlink, causing its file path stripping to fail. This results in
requests that look like:

https://example.com/wp-content//nix/store/...plugin/path/some-file.js

Since hard linking directories is not allowed, copying is the next best thing.

(cherry picked from commit 9d2c915)