Add SINGULARITY env var to nightly

Author: Derek Hower

.github/workflows/nightly.yml

@@ -26,6 +26,8 @@ jobs:
     needs: check_date
     if: ${{ needs.check_date.outputs.should_run != 'false' }}
     runs-on: ubuntu-latest
+    env:
+      SINGULARITY: 1
     steps:
       - name: Clone Github Repo Action
         uses: actions/checkout@v4