Use LLVM for Linux test unwinding

.github/workflows/test.yml

@@ -88,6 +88,8 @@ jobs:
 
       - name: "Cargo test"
         env:
+          # The Linux test suite exercises catch_unwind, which currently requires LLVM.
+          RUSTFLAGS: -Zcodegen-backend=llvm
           # Retry more than default to reduce flakes in CI
           UV_HTTP_RETRIES: 5
           RUST_BACKTRACE: 1