Make GH actions clone from the correct fork and cleanup

Signed-off-by: Šimon Brandner <simon.bra.ag@gmail.com>

.github/workflows/develop.yml

@@ -10,6 +10,8 @@ on:
 jobs:
     end-to-end:
         runs-on: ubuntu-latest
+        env: 
+          PR_NUMBER: ${{github.event.number}}
         container: vectorim/element-web-ci-e2etests-env:latest
         steps:
             - name: Checkout code