Say where the Lance worker comes from now that it ships
It is in the SeaweedFS image as worker-rust and on each release as
weed-worker_linux_{amd64,arm64}.tar.gz, so a Rust toolchain is no longer the
only way in. The binary is weed-worker, not weed-lance-worker: the crate keeps
the lance name, the binary does not.
Also records what the build needs from protoc - core vendors its own, lance's
crates need one of their own.
Chris Lu