Due to an incorrect slice allocation, the environment variable list was created with an empty string
element for each real element added.
It appears that this was silently ignored on Unix, but caused the following environment settings
to be ignored altogether on Windows.
This new argument allows overriding of the working directory of the child process, with the default still being the working directory of Terraform itself.
If the shell spawns a subprocess which doesn't close the output file
descriptors, the exec.Cmd will block on Wait() (see
golang.org/issue/18874). Use an os.Pipe to provide the command with a
real file descriptor so the exec package doesn't need to do the copy
manually. This in turn may block our own reading goroutine, but we can
select on that and leave it for cleanup later.
Andre Bindewald