Revert "Let's try 8 threads again..." - nope, memory/thread is too low

This reverts commit b9b95d10.

.gitlab-ci.yml

@@ -27,9 +27,9 @@ stages:
     stage: build
     script:
         # all cores including hyperthreading
-        - export NUM_CORES=$(grep "core id" /proc/cpuinfo | wc -l)
+#        - export NUM_CORES=$(grep "core id" /proc/cpuinfo | wc -l)
 #       # all pyhsical cores
-#        - export NUM_CORES=$(grep "core id" /proc/cpuinfo | sort -u | wc -l)
+        - export NUM_CORES=$(grep "core id" /proc/cpuinfo | sort -u | wc -l)
         - export MAKEFLAGS="-l$(echo 1.5*$NUM_CORES | bc) -j$NUM_CORES"
         - mkdir -p "./builddir/$CI_JOB_NAME"
         - pushd "./builddir/$CI_JOB_NAME"