ci: install diffusers from PyPI inside Docker image

The Docker image provides system deps (libGL, etc.) but ships a dev
version from main with no matching tag. Override with PyPI release
inside the container so we test against what real users have, and the
tag clone works.

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

Author: Titus-von-Koeller

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

@@ -166,9 +166,10 @@ jobs:
 
       - uses: actions/checkout@v4
 
-      - name: Install bnb (from continuous-release) over the image's version
+      - name: Install bnb + diffusers from PyPI (overriding image versions)
         run: |
-          pip install --force-reinstall --no-deps ${BNB_WHEEL_URL}
+          pip install "bitsandbytes[test] @ ${BNB_WHEEL_URL}"
+          pip install diffusers transformers accelerate peft
 
       - name: Clone diffusers matching installed version
         run: |