Honour CI constraints in Neko tests (#10373)

Ideally we'd be running the Neko tests against the requirements that
will actually form the complete package.  In practice, however, there
are persistent failures from Neko stemming from PySCF's use of SciPy,
which we get via Nature, which is no longer under Qiskit control.

While we work out what the future of the Neko test suite is, with the
application modules now no longer being IBM-managed Qiskit packages, we
can apply the CI constraints so we can return to getting reports from
the parts of the integration tests that are actually within our control.

(cherry picked from commit df5d413)

.github/workflows/neko.yml

@@ -15,3 +15,4 @@ jobs:
       - uses: Qiskit/qiskit-neko@main
         with:
           test_selection: terra
+          repo_install_command: "pip install -c constraints.txt ."