Update GitHub Actions

.github/workflows/main.yml

@@ -8,10 +8,11 @@ jobs:
     - name: Run tests
       run: |
         rm -rf ~/.conda/envs/miplearn-test
-        yes | conda create --name miplearn-test python=3.8
+        yes | conda create --name miplearn-test python=3.6
         julia -e 'using Pkg; Pkg.add(PackageSpec(url="https://github.com/iSoron/CPLEXW.git")); Pkg.build("CPLEXW")'
         julia -e 'using Pkg; Pkg.add(PackageSpec(url="https://github.com/iSoron/TinyBnB.jl.git"))'
         (cd /opt/gurobi900/linux64 && ~/.conda/envs/miplearn-test/bin/python setup.py install)
+        (cd /opt/cplex-12.8/cplex/python/3.6/x86-64_linux && ~/.conda/envs/miplearn-test/bin/python setup.py install)
         ~/.conda/envs/miplearn-test/bin/pip install -r requirements.txt
         ~/.conda/envs/miplearn-test/bin/pytest