fix: add defer pinner.Unpin()

[asteurer]

This fixes the following panic I'm getting when using async WIT functions:

 panic: runtime error: runtime.Pinner: found leaking pinned pointer; forgot to call Unpin()?

goroutine 6 [running]:
runtime.runFinalizers()
    /home/asteurer/.cache/componentize-go/v2/go-linux-amd64-bootstrap/src/runtime/mfinal.go:272 +0x5e
error while executing at wasm backtrace:
0: 0x11d931 - <unknown>!runtime.abort
1: 0x95f45 - <unknown>!runtime.fatalpanic
2: 0x113d55 - <unknown>!runtime.gopanic
3: 0x1201a8 - <unknown>!wasm_pc_f_loop_export
4: 0x18c0de - <unknown>!_async_lift_my_func

@dicej