Bug 1543662 Only check mar channel IDs for destination mars r=mtabara

Generating partials from betas doesn't work if we check the 'from' mar channel IDs, as we don't know what's valid for those.

Differential Revision: https://phabricator.services.mozilla.com/D27866

--HG--
extra : moz-landing-system : lando

taskcluster/docker/funsize-update-generator/scripts/funsize.py

@@ -349,7 +349,8 @@ async def manage_partial(partial_def, filename_template, artifacts_dir,
         with ddstats.timer('mar.unpack.time'):
             await unpack(work_env, dest, unpack_dir)
 
-        check_channels_in_files.append(dest)
+        if mar_type == 'to':
+            check_channels_in_files.append(dest)
 
         if mar_type == 'from':
             version = get_option(unpack_dir, filename="application.ini",