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.

PROTOCOL.md

@@ -265,6 +265,11 @@ The reply is `{"type": "built", "wasm": "/actor/cargo/public/builds/<n>/<name>.w
 or `{"type": "build_failed", "errors": "...", "tests": {...}}`. Pass
 `wasm`, `source`, and `built_by` on to `spawn` or `upgrade`.
 
+Build scripts can compile C for `wasm32-wasip2`: the host sets
+`CC_wasm32_wasip2` and `AR_wasm32_wasip2` to a WASI clang with wasi-libc, so
+the `cc` crate (for example `rusqlite` with `bundled`) works. Its headers are
+WASI 0.1: C code that needs sockets does not build.
+
 A Rust actor depends on the SDK with `sausage-actor = "0.1"`. `cargo` points
 that dependency at the SDK of the host, which it publishes at
 `/actor/cargo/public/sdk/`.