Bug 1858159 - Fix attribution-l10n beetmover upstream task reference on nightly r=bhearsum,taskgraph-reviewers

Differential Revision: https://phabricator.services.mozilla.com/D190626

taskcluster/gecko_taskgraph/manifests/firefox_nightly.yml

@@ -357,6 +357,8 @@ mapping:
         <<: *default
         description: "Main package disk image for Mac OS X platforms"
         all_locales: true
+        # Attribution-l10n jobs don't have locale in the artifact path
+        source_path_modifier: ""
         from:
             - attribution
             - attribution-l10n