Native benchmarks rely on hard-coded path

When running a native benchmark that is not named `benchmark.so`, the native engine will fail due to this hard-coded address:

https://github.com/bytecodealliance/sightglass/blob/04546548e60dcce4b6691f8ddb75442c4d251ad9/engines/native/libengine/src/lib.rs#L285

Previously this was just a paper cut, only affecting in-development benchmarks (one would expect the native engine to open `your-benchmark-path.so` but it would not) but now that multiple benchmarks can live in a single directory, the path given to the `sightglass-cli benchmark` command should be the one communicated to the native engine. (Some thoughts: (a) add a new field to the `WasmBenchConfig` passed in to the native engine in `wasm_bench_create`, (b) hack the `wasm_bytes` passed to `wasm_bench_compile` to contain something like `NATIVE_PATH###/path/passed/to/sightglass.so` instead of the bytes of the shared library, which are useless.)