Add wasm32-wasip2 to build-manifest tool

This is an accidental omission of mine from rust-lang#126967 which means that
`rustup target add wasm32-wasip2` isn't working on today's nightlies.

Author: alexcrichton

src/tools/build-manifest/src/main.rs

@@ -157,6 +157,7 @@ static TARGETS: &[&str] = &[
     "wasm32-wasi",
     "wasm32-wasip1",
     "wasm32-wasip1-threads",
+    "wasm32-wasip2",
     "x86_64-apple-darwin",
     "x86_64-apple-ios",
     "x86_64-fortanix-unknown-sgx",