fix for export outside of default env path

[jerowe]

Fixes issue https://github.com/conda/conda-env/issues/241

#without a name supplied name is the basename of the prefix
conda env export -p /path/to/my_env

#Or supply a name
conda env export --path /path/to/my_env --name with_this_name

[kalefranz]

conda-env code moved to the conda proper repository a while back. I left PRs open so people could port them over. Obviously, I could still do better at communicating.

If you want to pull this over to the conda repo, it'll be more straight-forward to take from there :)

[jerowe]

Happy to do so, but I have another PR open, and its failing. I'm not sure if its failing because of me or because of something else. Once I get that figured out (and I'd appreciate input!) I'll be happy to open the PR in conda proper ;-)