Fix par-map&

To actually work in parallel.

guix-data-service/utils.scm

@@ -479,16 +479,18 @@ available.  Return the resource once PROC has returned."
 
 (define (par-mapper' mapper cons)
   (lambda (proc . lists)
-    (let loop ((lists lists))
-      (match lists
-        (((heads tails ...) ...)
-         (let ((tail (loop tails))
-               (head (defer-to-parallel-fiber
-                       (lambda ()
-                         (apply proc heads)))))
-           (cons (fetch-result-of-defered-thunks head) tail)))
-        (_
-         '())))))
+    (apply
+     fetch-result-of-defered-thunks
+     (let loop ((lists lists))
+       (match lists
+         (((heads tails ...) ...)
+          (let ((tail (loop tails))
+                (head (defer-to-parallel-fiber
+                        (lambda ()
+                          (apply proc heads)))))
+            (cons head tail)))
+         (_
+          '()))))))
 
 (define par-map& (par-mapper' map cons))