[CI] upload the forgejo under test as an artifact for end-to-end (squash) use forgejo-pr

cascade a PR to https://code.forgejo.org/forgejo/end-to-end on the
forgejo-pr branch which will only run end-to-end tests relevant to a
Forgejo PR and not the entire end-to-end suite.

(cherry picked from commit 011de92174)

.forgejo/workflows/cascade-setup-end-to-end.yml

@@ -62,7 +62,7 @@ jobs:
           destination-url: https://code.forgejo.org
           destination-fork-repo: cascading-pr/end-to-end
           destination-repo: forgejo/end-to-end
-          destination-branch: main
+          destination-branch: forgejo-pr
           destination-token: ${{ secrets.END_TO_END_CASCADING_PR_DESTINATION }}
           close-merge: true
           update: .forgejo/cascading-pr-end-to-end