Missing `exec`s

Steps to reproduce:
```
$ proctrace record -o demo.log -- ./demo_script.sh
$ proctrace render -i demo.log -d by-process
```

You'll see two processes that `fork` without an `exec`.
I'm pretty confident these are the `printf` calls in `demo_script.sh`.
It's possible that somehow the shell builtin `printf` is getting called,
but I tried to avoid that by calling `command -v printf` directly.