github: do not nest ${{}} inside condition

In commit 2596d157, we added a condition to run auto-backport.py only
when the GitHub Action is triggered by a push to the default branch.
However, this introduced an unexpected error due to incorrect condition
handling.

Problem:
- `github.event.before` evaluates to an empty string
- GitHub Actions' single-pass expression evaluation system causes
  the step to always execute, regardless of `github.event_name`

Despite GitHub's documentation suggesting that ${{ }} can be omitted,
it recommends using explicit ${{}} expressions for compound conditions.

Changes:
- Use explicit ${{}} expression for compound conditions
- Avoid string interpolation in conditional statements

Root Cause:
The previous implementation failed because of how GitHub Actions
evaluates conditional expressions, leading to an unintended script
execution and a 404 error when attempting to compare commits.

Example Error:

```
  python .github/scripts/auto-backport.py --repo scylladb/scylladb --base-branch refs/heads/master --commits ..2b07d93beac7bc83d955dadc20ccc307f13f20b6
  shell: /usr/bin/bash -e {0}
  env:
    DEFAULT_BRANCH: master
    GITHUB_TOKEN: ***
Traceback (most recent call last):
  File "/home/runner/work/scylladb/scylladb/.github/scripts/auto-backport.py", line 201, in <module>
    main()
  File "/home/runner/work/scylladb/scylladb/.github/scripts/auto-backport.py", line 162, in main
    commits = repo.compare(start_commit, end_commit).commits
  File "/usr/lib/python3/dist-packages/github/Repository.py", line 888, in compare
    headers, data = self._requester.requestJsonAndCheck(
  File "/usr/lib/python3/dist-packages/github/Requester.py", line 353, in requestJsonAndCheck
    return self.__check(
  File "/usr/lib/python3/dist-packages/github/Requester.py", line 378, in __check
    raise self.__createException(status, responseHeaders, output)
github.GithubException.UnknownObjectException: 404 {"message": "Not Found", "documentation_url": "https://docs.github.com/rest/commits/commits#compare-two-commits", "status": "404"}
```

Fixes scylladb/scylladb#21808
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

Closes scylladb/scylladb#21809

.github/workflows/add-label-when-promoted.yaml

@@ -49,7 +49,7 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.AUTO_BACKPORT_TOKEN }}
         run: python .github/scripts/label_promoted_commits.py  --commits ${{ github.event.before }}..${{ github.sha }} --repository ${{ github.repository }} --ref ${{ github.ref }}
       - name: Run auto-backport.py when promotion completed
-        if: github.event_name == 'push' && github.ref == 'refs/heads/${{ env.DEFAULT_BRANCH }}'
+        if: ${{ github.event_name == 'push' && github.ref == format('refs/heads/{0}', env.DEFAULT_BRANCH) }}
         env:
           GITHUB_TOKEN: ${{ secrets.AUTO_BACKPORT_TOKEN }}
         run: python .github/scripts/auto-backport.py --repo ${{ github.repository }} --base-branch ${{ github.ref }} --commits ${{ github.event.before }}..${{ github.sha }}
@@ -65,7 +65,7 @@ jobs:
             echo "backport_label=false" >> $GITHUB_OUTPUT
           fi
       - name: Run auto-backport.py when label was added
-        if: github.event_name == 'pull_request_target' && steps.check_label.outputs.backport_label == 'true' && (github.event.pull_request.state == 'closed' && github.event.pull_request.merged == true)
+        if: ${{ github.event_name == 'pull_request_target' && steps.check_label.outputs.backport_label == 'true' && github.event.pull_request.merged == true }}
         env:
           GITHUB_TOKEN: ${{ secrets.AUTO_BACKPORT_TOKEN }}
         run: python .github/scripts/auto-backport.py --repo ${{ github.repository }} --base-branch ${{ github.ref }} --pull-request ${{ github.event.pull_request.number }} --head-commit ${{ github.event.pull_request.base.sha }}