fix invocation of pip for nbconvert downstream (#1037)

Author: minrk

.github/workflows/downstream.yml

@@ -50,7 +50,7 @@ jobs:
       - uses: jupyterlab/maintainer-tools/.github/actions/downstream-test@v1
         with:
           package_name: nbconvert
-          package_spec: pip install -e ".[test]"
+          package_spec: -e ".[test]"
 
   jupyter_server:
     runs-on: ubuntu-latest