fix: build-release workflow stops its own end-to-end checks when run concurrently (#10632)

`build-release.yml` attempts to run an end-to-end check with a cascading PR, but it doesn't target the currently building branch.  When two releases build simultaneously (eg. `forgejo/v14.0` and `forgejo`), whichever one starts the end-to-end test first is then "cancelled" by the second one as it pushes an update to the same branch.

This will be a bit of an experimental change due to the difficulty in setting up a test environment.  After merge, I intend to watch a v14 and forgejo build and verify that they are independent, and, that both are actually tested with the correct target build.

This introduces a need to backport any changes to `.forgejo/cascading-release-end-to-end` in the future to maintain cascading functionality in all active releases.

## Checklist

The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).

### Tests

- I added test coverage for Go changes...
  - [ ] in their respective `*_test.go` for unit tests.
  - [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [x] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/10632
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Co-authored-by: Mathieu Fenniak <mathieu@fenniak.net>
Co-committed-by: Mathieu Fenniak <mathieu@fenniak.net>

.forgejo/cascading-release-end-to-end

@@ -2,6 +2,9 @@
 
 set -ex
 
+# WARNING: Changes to the behaviour of this file should be backported to all active releases, as it is used in
+# `build-release.yml` from release branches.
+
 end_to_end=$1
 end_to_end_pr=$2
 forgejo=$3

.forgejo/workflows/build-release.yml

@@ -206,7 +206,7 @@ jobs:
           origin-url: ${{ env.GITHUB_SERVER_URL }}
           origin-repo: ${{ github.repository }}
           origin-token: ${{ secrets.CASCADE_ORIGIN_TOKEN }}
-          origin-ref: refs/heads/forgejo
+          origin-ref: ${{ github.ref }}
           destination-url: https://code.forgejo.org
           destination-fork-repo: ${{ vars.CASCADE_DESTINATION_DOER }}/end-to-end
           destination-repo: forgejo/end-to-end