Implement --prune flag for mamba env update

[adament]

This fixes #688 and the similar issue for conda conda/conda#7279.

The prune flag is implemented by running the solver as if it was given
an empty prefix and then computing the diff between the current prefix
and the newly solved target and executing those resulting links and
unlinks.

This assumes that it is always possible to transform a given prefix to
any other consistent set of packages, since the solver never sees the
list of currently installed packages.

Furthermore a small fix for leaking temporary files is included.

This pull request is not compatible with the current conda implementation but instead implements the feature as documented in conda env update --help and as requested in the related conda issue (as far as I understand it). As it was suggested in #688 it might thus be preferred to implement this as a separate argument to mamba env update but with my very limited understanding of conda internals it seems a much greater task.

[adament]

Off topic question but is there a way I can run the python formatters locally?

[baszalmstra]

Off topic question but is there a way I can run the python formatters locally?

I simply run pre-commit run --all, but you can also simply install and run black=19.10b

[adament]

Off topic question but is there a way I can run the python formatters locally?

I simply run pre-commit run --all, but you can also simply install and run black=19.10b

Thank you!

[wolfv]

Thanks!