community, langchain, infra: store extended test deps outside of poetry (#18995)

poetry can't reliably handle resolving the number of optional "extended
test" dependencies we have. If we instead just rely on pip to install
extended test deps in CI, this isn't an issue.

.github/workflows/check_diffs.yml

@@ -116,7 +116,8 @@ jobs:
         shell: bash
         run: |
           echo "Running extended tests, installing dependencies with poetry..."
-          poetry install -E extended_testing --with test
+          poetry install --with test
+          poetry run pip install -r extended_requirements.txt
 
       - name: Run extended tests
         run: make extended_tests