Fix cached runtime embedding across feature builds

[ludfjig]

Copy the selected runtime into OUT_DIR and use that copy for embedding.

Default and trace_guest builds share one runtime output path. A trace_guest build can overwrite that file while Cargo retains the default build script's cached output. A later default library compilation then embeds the trace-enabled runtime.

Each configuration's OUT_DIR copy preserves its runtime. The nested Cargo target directory remains shared to reuse compiled dependencies. Custom runtimes supplied through HYPERLIGHT_JS_RUNTIME_PATH retain their existing environment and source-file tracking.

Original Reproduction

cargo build --release -p hyperlight-js
cargo build --release -p hyperlight-js --features trace_guest
cargo build --release -p hyperlight-js

Before this fix, the third build embedded the trace-enabled runtime despite requesting default

Uses the same approach as in in hyperlight-dev/hyperlight-wasm#551.

Fixes #296

[jprendes]

Could we instead copy it alongside the original runtime binary but appending a hash, something like hyperlight-wasm-runtime-xXxXxX, where we compute the hash from the features, profile, and crate source path?