common: dereference $dbus_socket correctly

Regression in e562c672 (D3663).

Bug: https://phabricator.apertis.org/T2215



Reviewed-by: Luis Araujo <luis.araujo@collabora.co.uk>
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D3764

common/common.sh

@@ -160,7 +160,7 @@ ensure_dbus_session() {
     local dbus_socket
 
     dbus_socket="/run/user/$UID/bus"
-    if [[ ! -e {dbus_socket} ]]; then
+    if [[ ! -e "${dbus_socket}" ]]; then
         cry "Could not find session bus..."
         return 1
     fi