Fix CI: set OMPI_MCA_rmaps_base_oversubscribe=1 to allow 4 ranks on 2-core runners

Author: sbryngelson

.github/workflows/convergence.yml

@@ -4,6 +4,9 @@ on:
   push:
   workflow_dispatch:
 
+env:
+  OMPI_MCA_rmaps_base_oversubscribe: 1
+
 jobs:
   convergence-1d:
     name: "1D Advection Convergence"