[auth] Adjust the email regexp to match luci-py's.
[auth] Adjust the email regexp to match luci-py's.

Otherwise luci-py pushes AuthDB proto that doesn't pass luci-go's
validation, which is bad.

R=a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org, ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org
BUG=953938

Change-Id: I08c96dd35c7f92c3177843ea12cdfe8b3d5cee30
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1577709
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

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