export: Fix exporting with an output file at a relative path.

Fixes #7051.

zerver/management/commands/export.py

@@ -111,6 +111,8 @@ class Command(ZulipBaseCommand):
         output_dir = options["output_dir"]
         if output_dir is None:
             output_dir = tempfile.mkdtemp(prefix="/tmp/zulip-export-")
+        else:
+            output_dir = os.path.realpath(output_dir)
         if os.path.exists(output_dir):
             shutil.rmtree(output_dir)
         os.makedirs(output_dir)