TCLOUD-4780: Try referring to env vars just by their name

Author: tiny-james

.github/workflows/preview_create.yml

@@ -68,7 +68,7 @@ jobs:
         build: ./build/
         redirects: ./redirects.json
         bucket: tiny-cloud-antora-docs-preview
-        prefix: ${{ vars.PR }}/${{ vars.RUN }}
+        prefix: ${{ PR }}/${{ RUN }}
         parallel: 10
 
     - name: Update pointer to current run output # put in the docs subfolder as we rely on that path in redirects