network/connection-editor: always load all available VPN plugins

Previously, when a list of VPN plugins was loaded once, it will not load
again the next time you try to add a VPN connnection. This causes an
issue that if you keep gnome-control-center open and install a new VPN
plugin, the new VPN plugin won't show up in the VPN plugin list.

Fix that by always load all avilable VPN plugins when showing the
"Add VPN" dialog.
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