cbuildbot: archive the manifest we used

Since the exact manifest a bot used to build is useful to developers,
add a step to do that in the ArchiveStage.  The file is tiny so it
shouldn't really impact things.

TEST=Set the snapshot_manifest parameter and make
     sure that buildbot makes manifest snapshots

Change-Id: Ib0a440fcac25a74aed74742b6a20a8a71b34595c
Reviewed-on: https://chromium-review.googlesource.com/49825
Reviewed-by: David James <davidjames@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed