pr_labeler: fix error in pyproject.toml (#2432)

The correct key-name is `build-backend`, not `backend`.

Author: gotmax23

hacking/pr_labeler/pyproject.toml

@@ -1,6 +1,6 @@
 [build-system]
 requires = ["setuptools"]
-backend = "setuptools.build_meta"
+build-backend = "setuptools.build_meta"
 
 [project]
 name = "ad-internal-pr-labeler"