Increase ulimit in notebook test actions (#2464)

Hopefully fixes #2387. See https://ss64.com/bash/ulimit.html for more
information.

Author: frankharkins

Diff for: .github/actions/set-up-notebook-testing/action.yml

@@ -52,3 +52,8 @@ runs:
       run: |
         sudo apt-get update
         sudo apt-get install texlive-pictures texlive-latex-extra poppler-utils graphviz
+
+    - name: Set ulimit
+      # https://github.com/Qiskit/documentation/issues/2387#issuecomment-2523753374
+      shell: bash
+      run: ulimit -n 6000