Fix early exit hook wd change

[keithduncan]

An early exit hook would revert the shell to the bootstrap’s working directory which is the builds dir and not the specific build being worked on. This was because the beforeWd didn’t hold the value of the "before working directory" but instead os.Getwd().

To handle this case I have removed that otherwise unused variable from the ScriptWrapper struct, and we now detecting the hook early exit case based on an absence of an afterEnv and return an error. The bootstrap handles that category of error by continuing on without applying any changes since they are incomputable.

This matches the previous behaviour where we would: get a blank afterEnv and do nothing with it (since we didn’t support propagating removed vars up into the shell env), and get an empty string for the "new" working directory which because it wasn’t absolute would be appended to the CWD to get the exact same string back 🙈

If Changes() returns an error because it cannot read the afterEnv (distinct from a successfully read but empty afterEnv) we return an error and the bootstrap process aborts the job, same behaviour as before.