remove the need to re-compile when testing Python code changes

[jameslamb]

Right now, every run of make cluster-image and make notebook-image needs to re-compile lib_lightgbm. This adds a few minutes to image building.

When testing changes to lightgbm.dask, it's common to only need to change things on the Python side (which shouldn't require recompilation).

This PR proposes compiling lib_lightgbm one time and then re-using that when re-installing the Python package.

Other changes:

• use Dask version for tags, and allow use of -e DASK_VERSION to test against different versions
• remove pins on pandas and numpy