set tutorials main branch for diff

Author: adrn

.github/workflows/prs.yml

@@ -35,12 +35,14 @@ jobs:
       - name: Run all tutorials
         if: contains(github.event.pull_request.labels.*.name, 'Run all tutorials')
         run: |
+          export TUTORIALS_MAIN_BRANCH=origin/main
           make executeall
           make convertall
 
       # Otherwise, only run tutorials that have been modified
       - name: Run only modified tutorials
         if: "!contains(github.event.pull_request.labels.*.name, 'Run all tutorials')"
         run: |
+          export TUTORIALS_MAIN_BRANCH=origin/main
           make execute
           make convert