python3Packages.scipy: Limit OpenMP threads during check phase

The scipy tests already run in parallel and the openmp threads come on
top of that. That overloads machines very easily and therefore we
introduce a scaling limit to the number of openmp threads used.

https://www.openmp.org/spec-html/5.0/openmpse50.html

pkgs/development/python-modules/scipy/default.nix

@@ -54,6 +54,7 @@ buildPythonPackage rec {
   checkPhase = ''
     runHook preCheck
     pushd "$out"
+    export OMP_NUM_THREADS=$(( $NIX_BUILD_CORES / 4 ))
     ${python.interpreter} -c "import scipy; scipy.test('fast', verbose=10, parallel=$NIX_BUILD_CORES)"
     popd
     runHook postCheck