Add note about GraphBLAS from conda

Author: alugowski

.github/workflows/tests.yml

@@ -27,6 +27,8 @@ jobs:
 
     - name: Install GraphBLAS (ubuntu)
       if: contains(matrix.os, 'ubuntu')
+        # Install pre-built GraphBLAS from conda-forge because GraphBLAS packages in Ubuntu repos are too old to be useful.
+        # See LAGraph's workflow: https://github.com/GraphBLAS/LAGraph/blob/stable/.github/workflows/build.yml
       run: |
         mkdir -p grb
         cd grb