Determine if the build is in presubmit using build id

The current logic to determine if a build is pre or postsubmit is the
existence of the change_info file but this file also exists in
postsubmit, causing the build to break. Use the build ID as the
heuristic instead which should be more reliable.

Test: abtd run
Bug: 314171817
Change-Id: I9871eee71c0574583b528135dadd42e3f485a780

ci/build_test_suites.py

@@ -39,7 +39,7 @@ REQUIRED_MODULES = frozenset(
 def build_test_suites(argv):
   args = parse_args(argv)
 
-  if not args.change_info:
+  if not os.environ.get('BUILD_NUMBER')[0] == 'P':
     build_everything(args)
     return