Try disabling parallelism on tutorials job (#5866)

We're histting a relatively high tutorial job failure rate caused by
jupyter/jupyter_client#541 on certain notebooks we run in the tutorials
job. This appears to be caused by multiprocessing usage inside the
qiskit that print to stdout while running. To try and avoid this issue,
this commit disables parallelism in qiskit by setting the env var to do
that. The only concern is whether we have sufficient time budget to
execute the notebooks in CI.

azure-pipelines.yml

@@ -633,6 +633,8 @@ stages:
             set -e
             cd qiskit-tutorials
             sphinx-build -b html . _build/html
+          env:
+            QISKIT_PARALLEL: False
         - task: ArchiveFiles@2
           inputs:
             rootFolderOrFile: 'qiskit-tutorials/_build/html'