fix: create venv first

Author: maxrjones

.github/workflows/downstream.yml

@@ -85,6 +85,7 @@ jobs:
       - name: Install numcodecs with test-zarr-main group
         working-directory: numcodecs
         run: |
+          uv venv
           uv pip install meson-python
           uv sync --group dev --group test-zarr-main --no-build-isolation
           uv pip install --no-build-isolation -e .