ci: uninstall diffusers before PyPI install in Docker image

pip install diffusers inside the container was a no-op: the image has
0.38.0.dev0 which pip considers higher than the latest PyPI release
(0.37.1), so it skips the install. Uninstall first to force the
PyPI version.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Titus-von-Koeller

.github/workflows/tests-integration-nightly.yml

@@ -169,6 +169,7 @@ jobs:
       - name: Install bnb + diffusers from PyPI (overriding image versions)
         run: |
           pip install "bitsandbytes[test] @ ${BNB_WHEEL_URL}"
+          pip uninstall -y diffusers
           pip install diffusers transformers accelerate peft
 
       - name: Clone diffusers matching installed version