Exclude workflow changes from being backported

The backport script for the PRs does not have the permission to backport
PRs which include workflow changes. So, these PRs are excluded from
being automatically backported.

Failed CI run:

https://github.com/timescale/timescaledb/actions/runs/5387338161/
   jobs/9780701395

> refusing to allow a GitHub App to create or update workflow
> `.github/workflows/xxx.yaml` without `workflows` permission)

scripts/backport.py

@@ -409,6 +409,21 @@ def report_backport_not_done(original_pr, reason, details=None):
         report_backport_not_done(original_pr, "cherry-pick failed", details)
         continue
 
+    # We don't have the permission to modify workflows
+    changed_files = {file.filename for file in original_pr.get_files()}
+    changed_workflow_files = {
+        filename
+        for filename in changed_files
+        if filename.startswith(".github/workflows/")
+    }
+    if changed_workflow_files:
+        details = (
+            f"The PR touches a workflow file '{list(changed_workflow_files)[0]}' "
+            " and cannot be backported automatically"
+        )
+        report_backport_not_done(original_pr, "backport failed", details)
+        continue
+
     # Push the backport branch.
     git_check(f"push --quiet {target_remote} @:refs/heads/{backport_branch}")
 
@@ -423,7 +438,6 @@ def report_backport_not_done(original_pr, reason, details=None):
     # Do not merge the PR automatically if it changes some particularly
     # conflict-prone files that are better to review manually. Also mention this
     # in the description.
-    changed_files = {file.filename for file in original_pr.get_files()}
     stopper_files = changed_files.intersection(
         ["sql/updates/latest-dev.sql", "sql/updates/reverse-dev.sql"]
     )