Don't check for existing destination when performing archive dry run.

Change-Id: Ie30425e9ed1e55518dd9b6b4da3b7a9bc374e208
diff --git a/tools/archive.py b/tools/archive.py
index 687844a..3dc0445 100755
--- a/tools/archive.py
+++ b/tools/archive.py
@@ -136,7 +136,7 @@
     version = GetGitHash()
 
   destination = GetVersionDestination('gs://', version, is_master)
-  if utils.cloud_storage_exists(destination):
+  if utils.cloud_storage_exists(destination) and not options.dry_run:
     raise Exception('Target archive directory %s already exists' % destination)
   with utils.TempDir() as temp:
     version_file = os.path.join(temp, 'r8-version.properties')