Use `cargo -Zbindeps` for `spawn-task` example

[nspin]

Once it stabilizes, use the -Zbindeps feature to simplify the build process for the spawn-task example. Right now, it is passed the path of the spawn-task-child binary via an environment variable. Once -Zbindeps is stabilized or (otherwise becomes more appropriate to use), it can just depend on the child's binary artifact via Cargo.