CARGO_UNSTABLE_SPARSE_REGISTRY=true breaks dev env doc builds

I had CARGO_UNSTABLE_SPARSE_REGISTRY=true in my dev environment, and running cargo run -- build crate rustls 0.20.0 failed with a lot of errors like:

[INFO] [stderr] warning: spurious network error (1 tries remaining): [6] Couldn't resolve host name (Could not resolve host: crates.io)

This is because we fetch dependencies outside the rustwide container, and make them available (via filesystem) inside the container. In my setup, CARGO_UNSTABLE_SPARSE_REGISTRY=true was being observed outside the container, but of course the container does not inherit the environment variable. So cargo inside the container was not seeing the prefetched files and was trying to fetch them.

The big fix here would be to do the fetching inside its own container, which does have network access (unlike the building container).

A couple of smaller fixes we could make:

• Pass --offline to cargo inside the container so it fails less opaquely if it would need to access the network.
• Filter all CARGO_* vars from the env before calling cargo.

Start with the cargo run -- build crate rustls 0.20.0 entry point and trace dependency fetching and environment propagation across the rustwide containers. Compare the proposed approaches—fetching in a networked container, passing --offline, or filtering CARGO_* variables—and confirm the build no longer attempts unavailable network access while using prefetched dependencies.