[roll] Roll fuchsia [pb] Concurrent downloading for all the artifacts

With local testing from my cloudtop, the downloading of whole PB is
reduced from 4min to ~15seconds

```
$ time ffx product download gs://fuchsia-public-artifacts-release/builds/8777219162463666449/transfer.json ~/local_pb --force
Progress for "Transfer download"
  complete
    2 of 2 steps (100.00%)

real	0m12.231s
user	0m6.689s
sys	0m4.001s
```

Original-Bug: b/288185927
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/878454
Original-Revision: 432b61ae0ef6f42d22ea291ab614de4985460993
GitOrigin-RevId: 153fe0d06f25f8d5576ffa93a1794e09fb28064f
Change-Id: Ib061a2c30b3518b8e22f7c7803cd949f23f957f1
diff --git a/stem b/stem
index 0429d71..1e9a619 100644
--- a/stem
+++ b/stem
@@ -11,7 +11,7 @@
              githooks="integration/git-hooks"
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="74dd73e7668fa0bd093ffa109c0ec0abf2450807"/>
+             revision="432b61ae0ef6f42d22ea291ab614de4985460993"/>
   </projects>
   <hooks>
     <hook name="install-environment"