[Telemetry] Only call _UpdatePageSetArchivesIfChanged once per PageSet.

This saves about 5s on my MBP. Hard to say what the savings'll be on the bots.
This might also help reliability.

BUG=388256

Review URL: https://codereview.chromium.org/476373002

Cr-Commit-Position: refs/heads/master@{#290124}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290124 0039d316-1c4b-4281-b951-d872f2087c98

tools/telemetry/telemetry/page/page_runner.py

@@ -296,6 +296,7 @@ def _PrepareAndRunPage(test, page_set, expectations, finder_options,
       logging.warning(str(e))
 
 
+@decorators.Cache
 def _UpdatePageSetArchivesIfChanged(page_set):
   # Attempt to download the credentials file.
   if page_set.credentials_path: