Reduce parallelism for qpy tests (#13735)

In #13506 the QPY backwards compatibility tests are failing because the
image is running out of disk space and github actions is killing the
runner. The qpy files themselves should be fairly small and will be
dwarfed by the virtual environments created to install old versions of
qiskit to generate old QPY payloads. We use GNU parallel to speed up the
testing, however by doing this we end up having multiple venvs at once
which increases the disk space usage. This commit attempts to reduce the
pressure on the disk space usage by decreasing the parallelism from N
cpus (which is currently 4 for linux runners according to [1]) to 2.
This will increase the runtime for this job but if we have insufficient
disk space on the image to generate new qpy files there isn't really
a choice.

[1] https://docs.github.com/en/actions/using-github-hosted-runners/using-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories

test/qpy_compat/run_tests.sh

@@ -42,7 +42,7 @@ python -m venv "$qiskit_venv"
 "$qiskit_venv/bin/pip" install -c "$repo_root/constraints.txt" "$qiskit_dev_wheel" packaging
 
 # Run all of the tests of cross-Qiskit-version compatibility.
-"$qiskit_python" "$our_dir/get_versions.py" | parallel --colsep=" " bash "$our_dir/process_version.sh" -p "$qiskit_python"
+"$qiskit_python" "$our_dir/get_versions.py" | parallel -j 2 --colsep=" " bash "$our_dir/process_version.sh" -p "$qiskit_python"
 
 # Test dev compatibility with itself.
 dev_version="$("$qiskit_python" -c 'import qiskit; print(qiskit.__version__)')"