Revert "Revert "Do not overcommit so much on CPU""

This reverts commit f459bc3.

It turns out that overcommitting makes things faster. I guess this generally
makes sense. We need to account for IO blocks, so stopping at CPUs-.2 is
probably too conservative. The load quickly drops and then we have too few
processes.

.ci/setup-make-parallelity.sh

@@ -20,7 +20,7 @@ set -ex
 # too high can lead to RAM being insufficient, so it's best to set the NTHREADS
 # variable manually in your CI configuration.
 if [ -z "$CPUTHREADS" ]; then
-    CPUTHREADS=$((`grep -E '^processor' /proc/cpuinfo | wc -l`+1))
+    CPUTHREADS=`grep -E '^processor' /proc/cpuinfo | wc -l`
 fi
 if [ -z "$RAMTHREADS" ]; then
     RAMTHREADS=$(( `grep MemTotal /proc/meminfo | awk '{ print $2 }'` / 1024 / 1024 / 2 ))