For ease of distinguishing a series of build from others in Goma,
let me make autoninja set unique build ID.
The ID can be usable for statistics purpose to see how much time is
usually spent for each build.

Since the unique ID is not specific to Goma, i.e. anybody invoked by
ninja can use it to distinguish builds, I chose AUTONINJA_BUILD_ID.
Since we do not need to distinguish each node, I chose uuid4.

Bug: b/77176746
Change-Id: I0b6a67811b420c51d1fd3fd113bf7f039a41e2ab
Reviewed-on: https://chromium-review.googlesource.com/c/1275685Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Fumitoshi Ukai <ukai@chromium.org>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>