Use PACKER_CACHE_DIR environment variable if set

[Wenzel]

Hi,

I'm using Packer in multiple projects on my computer, and malboxes is one of them.
As such, i have a shared ~/packer_cache directory, where all my MSDN ISOs are stored.

I'm always using packer like this

export PACKER_CACHE_DIR=/home/tarrma/.packer_cache
packer build template.json

in Malboxes you are already overriding this variable by DIRS.user_cache_dir
https://github.com/GoSecure/malboxes/blob/master/malboxes/malboxes.py#L326

Could you modify that behavior so that the PACKER_CACHE_DIR environment variable is checked first, and then defaulting to DIRS.user_cache_dir ?

Thanks.

[obilodeau]

Fixed by merging #100.