Can't use system protoc, node and qt when building

There seems to be no way to bypass the “download_and_extract” step in the build process.

I’m trying to write an ebuild for it because the upstream version is apparently buggy ( https:/ / ) and disabling sandbox sounds like a terrible idea.

However, there does not seem to be a way to use system dependencies for those out of cargo.

setup_python() provides a way to specify a local python binary, and a similar approach could be taken for the other items. For Qt, perhaps a venv that skipped the pip install lines and relied on the packages already being available with the provided binary could work. Happy to accept a clean PR that makes such changes.