android-repo-download: Use parallel-job-count instead of hard-coding job

count.

* guix/build/android-repo.scm (android-repo-fetch): Use parallel-job-count
instead of hard-coding job count.

guix/build/android-repo.scm

@@ -56,7 +56,8 @@ (define* (android-repo-fetch manifest-url manifest-revision directory
     (with-directory-excursion directory
       (invoke git-repo-command "init" "-u" manifest-url "-b" manifest-revision
               "--depth=1")
-      (invoke git-repo-command "sync" "-c" "--fail-fast" "-v" "-j" "3")
+      (invoke git-repo-command "sync" "-c" "--fail-fast" "-v" "-j"
+              (number->string (parallel-job-count)))
 
       ;; Delete vendor/**/.git, system/**/.git, toolchain/**/.git,
       ;; .repo/**/.git etc since they contain timestamps.