CI: Revert change to env var name

Author: eschnett

.github/workflows/UnitTests.yml

@@ -556,7 +556,7 @@ jobs:
       MV2_SMP_USE_CMA: 0
       # Work around issue with affinity not set.  Ref:
       # https://github.com/JuliaParallel/MPI.jl/pull/810#issuecomment-1920255386
-      MV2_ENABLE_AFFINITY: 0
+      MVP_ENABLE_AFFINITY: 0
 
     steps:
     - name: Checkout