Dev install instructions: Switch the rmg_env to libmamba solver.

The solver setting is environment-specific. If you set it before
you make the rmg_env then it is not set IN the rmg_env and when
you later do stuff to that environment, it's super slow.

Author: rwest

documentation/source/users/rmg/installation/anacondaDeveloper.rst

@@ -103,6 +103,10 @@ Installation by Source Using Anaconda Environment for Unix-based Systems: Linux
 
     conda activate rmg_env
 
+#. Switch the conda solver to libmamba again, to accelerate any changes you might make to this conda environment in the future::
+
+    conda config --set solver libmamba
+
 #. Compile RMG-Py after activating the conda environment ::
 
     make