scripts: Fix an issue in `purge-old-deployments` script.

We were not including the real path of the symlinks due to which we
were incorrectly deleting deployments pointed by last/current/next.

scripts/lib/zulip_tools.py

@@ -213,6 +213,9 @@ def get_recent_deployments(threshold_days):
         except ValueError:
             # Always include deployments whose name is not in the format of a timestamp.
             recent.add(target_dir)
+            # If it is a symlink then include the target as well.
+            if os.path.islink(target_dir):
+                recent.add(os.path.realpath(target_dir))
     if os.path.exists("/root/zulip"):
         recent.add("/root/zulip")
     return recent