[roll] Roll fuchsia [build] Add enable_jobserver build argument.

Set this to true in `args.gn` to run the build using a
GNU Make jobserver pool of job slots to coordinate parallel
tasks among recursive builds.

When building the IDK, this launches all sub-builds at the
same time, instead of in a pool of depth 5, and still reduces
overall build time successfully.

Early measurements on CQ seem to indicate that jobserver mode
could be enabled by default on all builds without affecting them
but for now keep this feature disabled. Other CLs will enable
it for specific builder configurations, such as `sdk-core-linux`
or `sdk-bazel-linux` since they will benefit the most out of it.

Original-Bug: 42163397
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1132393
Original-Revision: a02d19b4cb3287781581793fae3b7ee3423f468d
GitOrigin-RevId: e29d3723c465904cd2207da679dab7d3c2f0fd91
Change-Id: I2b758d86a2dc8550ccdc39503dca8b75b1443e38

Author: David 'Digit' Turner

stem

@@ -11,7 +11,7 @@
              githooks="integration/git-hooks"
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="2b65d09a95fa4b7d9d0fd4f8cd35a0394b78173b"/>
+             revision="a02d19b4cb3287781581793fae3b7ee3423f468d"/>
   </projects>
   <hooks>
     <hook name="install-environment"