CI/CD: Unify 'build' packaging command

We already use the more modern "python -m build" during the deploy
stage; so we bring the 'build' stage in line with that.

.github/workflows/rtc-tools.yml

@@ -28,8 +28,10 @@ jobs:
     timeout-minutes: 30
     steps:
     - uses: actions/[email protected]
-    - run: pip install setuptools  # Required for Python 3.12.
-    - run: python setup.py sdist bdist_wheel
+    - run: |
+        python -m pip install --upgrade pip
+        pip install build
+    - run: python -m build
     - uses: actions/upload-artifact@v4
       if: success()
       with: