Use conda lock to make builds reproducible

[jooh]

At the moment we unnecessarily invoke the solver when installing the environment from neuroconda.yml. The environment is already constructed to be solvable, and all versions are pinned, so this step does little except find the current builds for each package. We could probably live without that.

Perhaps we can instead use conda-lock to construct a lock file. This basically boils things down to a list of paths, which can be installed far more quickly.

It would also make it possible to run the builds on OS X, since conda lock has a cross-solver.

The key limiting factor of this approach is that pip files are not supported. Until this is resolved I don't think this is workable for us.

conda/conda-lock#4