icp-sync-plugin/build.rs: warn instead of panic when wasm32-wasip2 is missing

[SAY-5]

Fixes #543.

crates/icp-sync-plugin/build.rs unconditionally ran cargo build --target wasm32-wasip2 for the test fixture and panicked the entire build when the target wasn't installed. Distribution toolchains (Homebrew, distro packagers) often don't ship wasm32-wasip2, so cargo install icp-cli failed for downstream packagers.

The fixture is only consumed by #[test] functions in runtime.rs. CARGO_CFG_TEST isn't set during build-script execution, so suggested gating doesn't actually distinguish test from non-test invocations. Instead: keep building when the target is available, but on failure emit a cargo:warning and set TEST_PLUGIN_WASM= (empty). Tests that need the fixture will surface a clearer error at test time; cargo install and cargo build succeed cleanly on toolchains without wasm32-wasip2 (e.g. the Homebrew Rust at Homebrew/homebrew-core#280458).

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[lwshang]

Thank you @SAY-5 for the fix and the thorough analysis — we really appreciate it!

We landed on a similar approach in #544, which was just merged. Closing this to avoid a double-merge. Thanks again!