Log Packer Version to Debug

Figured this would be helpful and remove some debugging / support steps.

Also, I was trying to figure out a way to get the architecture and OS of the binary in there.

One thing I looked at was `runtime.GOOS` and `runtime.GOARCH`, but that didn't seem to give me what I wanted.

Something like this:

`Packer Target OS/Architecture: Darwin x86_64`