Use POSIX `unset` rather than Busybox `env -u` for HAB_STUDIO_BINARY
Use POSIX `unset` rather than Busybox `env -u` for HAB_STUDIO_BINARY

On my Ubuntu 18.10 machine, overriding `HAB_STUDIO_BINARY` leads these
lines to fail with the following:

```
env: invalid number 'HAB_STUDIO_BINARY'
env: invalid number 'HAB_STUDIO_BINARY'
```

It appears to be something similar to this bug report:

http://lists.busybox.net/pipermail/busybox/2017-December/086002.html

To circumvent this, we'll just use `unset` to remove the variable from
the command environment (taking place in a subshell)

As an aside, `HAB_STUDIO_BINARY` seems to expect the given binary to
exist in a Habitat package, rather than being a bare script on the
workstation.

Signed-off-by: Christopher Maier <b4598f17a90ef80817bae8ad6c37249740caafb4@chef.io>

habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat