Never try to populate the LASTCHANGE file in non-standalone checkout

Since the build/ directory is checked out behind 'dawn_standalone', the
'lastchange' hook depending on that should be guarded by the same
condition.

Bug: dawn:437
Change-Id: If6d1f5ee505eab5bc5519ec6d29b2efe5d69d9aa
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/24123
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Jiajie Hu <jiajie.hu@intel.com>

DEPS

@@ -222,6 +222,7 @@ hooks = [
   {
     'name': 'lastchange',
     'pattern': '.',
+    'condition': 'dawn_standalone',
     'action': ['python', 'build/util/lastchange.py',
                '-o', 'build/util/LASTCHANGE'],
   },