Set software-sources access to read-only ipo the umask default

Software sources can be changed from within the Update Manager Edit menu. The revised sources are stored in /etc/apt/sources.list.d/official-package-repositories.list - with the default permissions as specified by the umask setting.
For systems whose umask setting prohibits read access by users which are neither the owner nor members of the file's group (root in this case), the update manager tray icon cannot show the update status, nor can the update manager show the status immediately following an update. Solution proposed is to set the other-user-access-permission to read-only (overriding the umask default) when writing this file.