Try to avoid CI segfault by forcing tests to be single-threaded

Author: leehart

.github/workflows/tests.yml

@@ -33,4 +33,10 @@ jobs:
         run: pip install "${{ matrix.numpy-version }}" .[dev]
 
       - name: Run unit tests
+        env:
+          OMP_NUM_THREADS: 1
+          OPENBLAS_NUM_THREADS: 1
+          MKL_NUM_THREADS: 1
+          VECLIB_MAXIMUM_THREADS: 1
+          NUMEXPR_NUM_THREADS: 1
         run: pytest -v tests --ignore tests/integration --typeguard-packages=malariagen_data,malariagen_data.anoph