Use `HYGEN_TMPLS` instead of symlinks

[roblafeve]

Until now we've depended on linking the _templates directories into the CWD, running the generator, then deleting it afterwards. This works, but creates a weird shift in the file tree while the generator is running. I think we can use the HYGEN_TMPLS variable to achieve our goal without the linking.

https://www.hygen.io/docs/faq#i-want-to-use-generators-from-a-single-place