Autodetect the need for conda create --copy

[natefoo]

As discussed in the comments on #3105, we could pretty easily detect whether conda create --copy is needed when setting up (even cached) job conda environments by attempting a hard link and then using --copy if it fails, and avoid admins having to explicitly set it.

That said, using symlinks instead of --copy should be safe as of Conda 4.3, as per conda/conda#3308 and conda/conda#3870.

[natefoo]

With mulled environments now we no longer make per-job conda env copies, so this is no longer needed.