This follows the same logic as the system bwrap.
Otherwise, build-time tests will fail when we are using a system
xdg-dbus-proxy and not compiling our own (the symptom is that testlibrary
hangs).
Signed-off-by: Simon McVittie <smcv@collabora.com>
Closes: #2823
Approved by: matthiasclasen
(cherry picked from commit 8115489116)