python3Packages.scikit-bio: reduce openmp threads during tests

The tests finish quickly enough with just a single OpenMP thread and this
prevents it from becoming a scheduling nightmare on hydra.

pkgs/development/python-modules/scikit-bio/default.nix

@@ -62,6 +62,10 @@ buildPythonPackage (finalAttrs: {
     pytestCheckHook
   ];
 
+  preCheck = ''
+    export OMP_NUM_THREADS=1
+  '';
+
   # only the $out dir contains the built cython extensions, so we run the tests inside there
   enabledTestPaths = [ "${placeholder "out"}/${python.sitePackages}/skbio" ];