shell/network-agent: Do not query keyring in greeter mode

shell/network-agent: Do not query keyring in greeter mode

When trying to connect to a network from gdm, it doesn't make sense to query
secrets from the gdm user since it's a system user.

Furthermore, gdm runs an isolated dbus-session per gnome-shell instance
(for multi-seat setups). Instead, gnome-keyring-daemon is started by systemd
and so it registers on the _main_ dbus session of the gdm user session.
Then, gnome-shell tries to dbus-activate another gnome-keyring-daemon on its
isolated bus, but gnome-keyring-daemon refuses to start as it sees another
instance already running, exposed at $XDG_RUNTIME_DIR/keyring/control.
After a 25s timeout, gnome-shell aborts the request without ever prompting
for a new password.

Because it is both problematic and pointless to query secrets in this case,
let's avoid it altogether and just prompt the user for the network password.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3646>