(zilch magic): configurable CA build job count

Change-Id: I6a6a6964f157a33b35dae7245c344be7907c5221

core/src/magic.sld

@@ -23,7 +23,7 @@
     store-path-realised store-path-open
     store-path-devirtualise
 
-    drv-resolve-ca
+    ca-thread-count drv-resolve-ca
 
     zilch-magic-counters)
 
@@ -333,6 +333,8 @@
    (define-record-printer (<pending-item> item out)
     (fprintf out "#<pending-item ~A - awaiting ~S>" (derivation-path (pending-item-ca-drv item)) (pending-item-awaiting-count item)))
 
+   (define ca-thread-count (make-parameter 4))
+
    (define (rewrite-ca-stack input-drv)
      ; A mapping of CA derivation path to <pending-item>.
      (define ca-to-pending-map (mapping (make-default-comparator)))
@@ -478,7 +480,7 @@
               (builder quit conn))))))
 
      (define builder-threads '())
-     (do ((i 0 (+ i 1))) ((>= i 16) #f)
+     (do ((i 0 (+ i 1))) ((>= i (ca-thread-count)) #f)
        (set! builder-threads
         (cons
          (thread-start!