store: Use a decaying cutoff in 'map/accumulate-builds'.

This reduces the wall-clock time of:

  ./pre-inst-env guix system vm gnu/system/examples/desktop.tmpl -n

from 2m13s to 53s (the timings depend on which derivations have already
been built and are in store; in this case, many were missing).

* guix/store.scm (default-cutoff): New variable.
(map/accumulate-builds): Use it.  Parameterize it in recursive calls to
have decaying cutoff.

guix/store.scm

@@ -1362,8 +1362,12 @@ object, only for build requests on EXPECTED-STORE."
         (unresolved things continue)
         (continue #t))))
 
+(define default-cutoff
+  ;; Default cutoff parameter for 'map/accumulate-builds'.
+  (make-parameter 32))
+
 (define* (map/accumulate-builds store proc lst
-                                #:key (cutoff 30))
+                                #:key (cutoff (default-cutoff)))
   "Apply PROC over each element of LST, accumulating 'build-things' calls and
 coalescing them into a single call.
 
@@ -1377,6 +1381,9 @@ CUTOFF is the threshold above which we stop accumulating unresolved nodes."
     (build-accumulator store))
 
   (define-values (result rest)
+    ;; Have the default cutoff decay as we go deeper in the call stack to
+    ;; avoid pessimal behavior.
+    (parameterize ((default-cutoff (quotient cutoff 2)))
       (let loop ((lst lst)
                  (result '())
                  (unresolved 0))
@@ -1391,7 +1398,7 @@ CUTOFF is the threshold above which we stop accumulating unresolved nodes."
              (obj
               (loop tail (cons obj result) unresolved))))
           (()
-         (values (reverse result) lst)))))
+           (values (reverse result) lst))))))
 
   (match (append-map (lambda (obj)
                        (if (unresolved? obj)