Set KHIOPS_PROC_NUMBER > 2 so that mpiexec gets set to non-empty string

Author: popescu-v

.github/workflows/unit-tests.yml

@@ -198,6 +198,8 @@ jobs:
         run: |
           pip install -r test-requirements.txt
       - name: Test Khiops Integration
+        env:
+          KHIOPS_PROC_NUMBER: 4
         shell: pwsh
         run: |
           # Refresh environment variables by using preexisting Chocolatey