build.rs: skip wasm32-wasip2 test fixture build for non-test builds

[chenrui333]

Problem

crates/icp-sync-plugin/build.rs unconditionally calls build_test_fixture(), which runs cargo build --target wasm32-wasip2 to compile a test fixture. This fails when the wasm32-wasip2 target is not installed:

error[E0463]: can't find crate for `core`
  = note: the `wasm32-wasip2` target may not be installed

thread 'main' panicked at crates/icp-sync-plugin/build.rs:33:5:
cargo build --target wasm32-wasip2 failed for test fixture

The TEST_PLUGIN_WASM env var set by this build step is only used in #[test] functions in runtime.rs, so the fixture is not needed for regular cargo build or cargo install.

Suggested fix

Gate build_test_fixture() behind a check so it only runs during test builds, e.g.:

fn main() {
    println!("cargo:rerun-if-changed=sync-plugin.wit");
    println!("cargo:rerun-if-changed=tests/fixtures/test-plugin/src/lib.rs");
    println!("cargo:rerun-if-changed=tests/fixtures/test-plugin/Cargo.toml");

    // Only build the wasm test fixture when running tests
    if std::env::var("CARGO_CFG_TEST").is_ok() {
        build_test_fixture();
    }
}

Context

This came up while packaging icp-cli 0.2.6 for Homebrew (Homebrew/homebrew-core#280458). The Homebrew Rust toolchain does not include the wasm32-wasip2 target. We are currently working around this with an inreplace in the formula.