ci: install pytest-xdist for transformers v5.x compatibility

Transformers v5.x registers a pytest plugin (NetworkDebugPlugin) that
uses the pytest_configure_node hook from pytest-xdist. Without xdist
installed, pluggy raises PluginValidationError during collection.

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

Author: Titus-von-Koeller

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

@@ -65,7 +65,7 @@ jobs:
 
       - name: Install transformers and clone matching tag
         run: |
-          pip install -U transformers accelerate
+          pip install -U transformers accelerate pytest-xdist
           TRANSFORMERS_VERSION=$(pip show transformers | awk '/^Version:/ {print $2}')
           echo "Installed transformers v${TRANSFORMERS_VERSION}"
           git clone --depth=1 --branch "v${TRANSFORMERS_VERSION}" \
@@ -120,7 +120,7 @@ jobs:
 
       - name: Install transformers and clone matching tag
         run: |
-          pip install -U transformers accelerate
+          pip install -U transformers accelerate pytest-xdist
           TRANSFORMERS_VERSION=$(pip show transformers | awk '/^Version:/ {print $2}')
           echo "Installed transformers v${TRANSFORMERS_VERSION}"
           git clone --depth=1 --branch "v${TRANSFORMERS_VERSION}" \