Restrict OpenMP to 1 thread

Otherwise, OpenMP thinks it is allowed to take as
many threads as there are cores, which leads to a
quadratic catastrophe when running tests in parallel.
OpenMP is used by Debian's installation of Suitesparse.

.gitlab-ci.yml

@@ -2,6 +2,7 @@
 variables:
   GIT_SUBMODULE_STRATEGY: recursive
   HYPERTHREADING: "false"
+  OMP_NUM_THREADS: 1
 
 before_script:
   - echo "Running make concurrently with ${DUNECI_PARALLEL} cores, where possibly"