Fix push issues for cleanup by removing single-commit

- note: in the future, `JamesIves/github-pages-deploy-action@4.1.5` may need to be replaced or supplemented by git commands to fully reset the preview branch so that images and builds do not linger and take up storage for too long

.github/workflows/preview_cleanup.yml

@@ -12,7 +12,7 @@ jobs:
         runs-on: ubuntu-latest
         if: github.event.label.name == 'use-preview'
         env:
-            BASE: ./refs/pull/${{ github.event.pull_request.number }}
+            BASE: refs/pull/${{ github.event.pull_request.number }}
 
         steps:
             - uses: actions/checkout@v2
@@ -28,7 +28,6 @@ jobs:
               with:
                   folder: .
                   branch: build-previews
-                  single-commit: true
 
             - name: send comment
               uses: marocchino/sticky-pull-request-comment@v2