Install ipykernel from source in the jupyter_kernel_test downstream job 🤖🤖

`pip install -e ".[test]"` in the jupyter_kernel_test clone resolved
ipykernel from PyPI, so the job tested the released wheel instead of
this checkout. Install the checkout first, and fail if the installed
ipykernel has no direct_url.json (i.e. came from an index).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: 2 people

.github/workflows/downstream.yml

@@ -97,5 +97,7 @@ jobs:
         run: |
           git clone https://github.com/jupyter/jupyter_kernel_test.git
           cd jupyter_kernel_test
+          pip install ..
           pip install -e ".[test]"
+          python -c "import importlib.metadata as m; assert m.distribution('ipykernel').read_text('direct_url.json'), 'ipykernel under test is not this checkout'"
           python test_ipykernel.py