Adjust system bus socket to /run/dbus/system_bus_socket

See https://salsa.debian.org/utopia-team/dbus/-/commit/e3a4c1aec4aa8eaedac0e1bddc383529c86b72c0



The interoperable cross-distro path is /var/run/dbus/system_bus_socket,
so this remains the upstream default for the benefit of distributions
where /var/run and /run are (problematically) not guaranteed to be
equivalent. However, Debian Policy since at least v4.1.5 guarantees
that /var/run is a symlink to /run, and this has been implemented for
several stable releases (since at least initscripts 2.88dsf-29 in 2012,
in the sysvinit case), so it is harmless to prefer the path in /run,
which has advantages in a few corner cases (ability to unmount /var is
the main one) and avoids warnings from systemd.

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>