Test downstream in parallel

Author: avik-pal

.github/workflows/Downstream.yml

@@ -50,6 +50,9 @@ jobs:
             @info "Not compatible with this release. No problem." exception=err
             exit(0)  # Exit immediately, as a success
           end
+        env:
+          RETESTITEMS_NWORKERS: 4
+          RETESTITEMS_NWORKER_THREADS: 2
       - uses: julia-actions/julia-processcoverage@v1
       - uses: codecov/codecov-action@v3
         with: