devel/glib20: unbreak session bus autolaunch after 94630ac169bc

Regressed by https://gitlab.gnome.org/GNOME/glib/-/commit/05ff2f877ca3

$ pkg install gtk4
$ service dbus onestart
$ cat /var/lib/dbus/machine-id
123456789012345678901234567890123
$ unset DBUS_SESSION_BUS_ADDRESS XDG_RUNTIME_DIR
$ gtk4-demo
Gdk-Message: Failed to get file transfer portal: Cannot spawn a message bus without a machine-id: Unable to load /var/local/lib/dbus/machine-id or /etc/machine-id: Failed to open file “/var/local/lib/dbus/machine-id”: No such file or directory
^C
$ gdbus call --session --dest=org.freedesktop.DBus --object-path=/ --method=org.freedesktop.DBus.ListNames
Error connecting: Cannot spawn a message bus without a machine-id: Unable to load /var/local/lib/dbus/machine-id or /etc/machine-id: Failed to open file “/var/local/lib/dbus/machine-id”: No such file or directory

PR: 257977
Tested by: Thibault Payet
Approved by: tcberner