[logdog] Log scopes as string in errors.
[logdog] Log scopes as string in errors.

The BQ schema for logs.archivist expects a string, not array, for
jsonPayload.fields.scopes. The auth package concatenates the scope array
to form a single string; make LogDog's scope logging consistent with
that behavior.

Change-Id: I3c8a361d845ee53de47df12ecc4792c67efc685c
Reviewed-on: https://chromium-review.googlesource.com/c/1452739
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>
Commit-Queue: Jao-ke Chin-Lee <5de1a387cc854ff3d04fab3d66d4e77a58511771@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go