cargo: compile C for wasm32-wasip2 with a WASI clang

The Nix package, the dev shell, and the test check set CC_wasm32_wasip2
and AR_wasm32_wasip2 to the clang of pkgsCross.wasi32 (wasi-libc), so
build scripts with the cc crate work, for example rusqlite with bundled
SQLite. The runtime-toolchain check links C into a probe with the
packaged host and an empty PATH.

cli/src/cargo_actor.rs

@@ -8,6 +8,9 @@
 //! The broker knows nothing about Rust. This actor is one way to get a
 //! `.wasm` file. Other actors can add other languages.
 //!
+//! Build scripts compile C for WASI with the clang in `CC_wasm32_wasip2`
+//! and `AR_wasm32_wasip2`. The Nix package and the dev shell set both.
+//!
 //! Actors depend on the SDK with `sausage-actor = "0.1"`. The SDK is not
 //! published: this actor copies the SDK of this host to `public/sdk/` (so
 //! that every actor can read it), and points the dependency there in the