evaluate: Create just as many threads as needed.

When using 'par-for-each', we'd spawn the whole thread pool of (ice-9
futures), with one thread per core.  Using 'n-par-for-each' allows us to
spawn just as many threads as needed.

* src/cuirass/scripts/evaluate.scm (cuirass-evaluate): Use
'n-par-for-each' instead of 'par-for-each'.

src/cuirass/scripts/evaluate.scm

@@ -116,7 +116,8 @@ registered in database."
              ;; speeds up the evaluation as the evaluations can be performed
              ;; concurrently.  It also decreases the amount of memory needed
              ;; per evaluation process.
-             (par-for-each
+             (n-par-for-each
+              (min (length systems) (current-processor-count))
               (lambda (system inferior)
                 (with-store store
                   (inferior-evaluation store inferior