Improve preview cleanup interpolation

.github/workflows/preview_cleanup.yml

@@ -11,6 +11,8 @@ jobs:
     clean:
         runs-on: ubuntu-latest
         if: github.event.label.name == 'use-preview'
+        env:
+            BASE: refs/pull/${{ github.event.pull_request.number }}
 
         steps:
             - uses: actions/checkout@v2
@@ -19,7 +21,7 @@ jobs:
                   persist-credentials: false
 
             - name: clean previews
-              run: 'rm -rf "./refs/pull/${{ github.event.pull_request.number }}"'
+              run: rm -rf "$BASE"
 
             - name: publish cleaned previews
               uses: JamesIves/github-pages-deploy-action@4.1.5