wasm: point wasmtime's cache at the writable test tmpdir

CI passes --test_env=HOME, and linux-sandbox mounts the agent's HOME
read-only, so wasmtime's default compilation cache hard-failed with
'failed to create cache directory'. Set XDG_CACHE_HOME to the test
tmpdir, and stop swallowing wasmtime's stderr so failures like this are
visible in the test log.

Claude-Session: https://claude.ai/code/session_01SmG1kqA3qB4WsLGU2xavuJ

Author: AttilaTheFun

examples/cross_compilation/wasm/reactor_test.sh

@@ -9,7 +9,12 @@ set -euo pipefail
 wasmtime="$TEST_SRCDIR/$1"
 reactor="$TEST_SRCDIR/$2"
 
-actual="$("$wasmtime" run --invoke greeting_length "$reactor" 2>/dev/null)"
+# wasmtime writes a compilation cache under XDG_CACHE_HOME (falling back to
+# $HOME), which can be read-only inside the test sandbox (e.g. when CI passes
+# --test_env=HOME); point it at the writable test tmpdir.
+export XDG_CACHE_HOME="${TEST_TMPDIR:-${TMPDIR:-/tmp}}"
+
+actual="$("$wasmtime" run --invoke greeting_length "$reactor")"
 if [[ "$actual" != "30" ]]; then
   echo "error: expected greeting_length to return 30, got: $actual" >&2
   exit 1