service: BuildPackages: Log completion of script

Log completion of building packages, the time the script took to run,
and the completion of the script.

BUG=b:218522022
TEST=CQ

Cq-Depend: chromium:3661663
Change-Id: Iee4e0d0dcb3fdb1847147c08c6fd29d4a71da1d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3661367
Commit-Queue: Cindy Lin <xcl@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Cindy Lin <xcl@google.com>
diff --git a/scripts/build_packages.py b/scripts/build_packages.py
index 213df26..ab84127 100644
--- a/scripts/build_packages.py
+++ b/scripts/build_packages.py
@@ -27,6 +27,7 @@
 from chromite.lib import cros_build_lib
 from chromite.lib import sysroot_lib
 from chromite.service import sysroot
+from chromite.utils import timer
 
 
 def build_shell_bool_style_args(parser: commandline.ArgumentParser,
@@ -371,6 +372,7 @@
   return parser, opts
 
 
+@timer.timed('Elapsed time (build_packages)')
 def main(argv: Optional[List[str]] = None) -> Optional[int]:
   commandline.RunInsideChroot()
   parser, opts = parse_args(argv)