Setting up CI in github workflows for a package using PyPlot

I am trying to set up CI in github workflows. I have followed the suggestions in The complication is that my package uses PyPlot. Following an example from somewhere (sorry, I forget where) I have specified the following in my ci.yml

       PYTHON: ""

(what does specifying an empty string here imply?)

CI completes successfully for ubuntu and windows but fails on maxOS with the following error

INTEL MKL ERROR: dlopen(/Users/runner/.julia/conda/3/lib/libmkl_intel_thread.1.dylib, 9): Library not loaded: @rpath/libiomp5.dylib
  Referenced from: /Users/runner/.julia/conda/3/lib/libmkl_intel_thread.1.dylib
  Reason: image not found.

Presumably similar to
Any suggestions on how I might correct this for CI?