This makes installed-tests (see commit b852b58f) do the right thing.
For build-time testing, spa/plugins/audioconvert/meson.build overrides
this with the SPA_PLUGIN_DIR environment variable, and for ad-hoc
testing by developers, pw-uninstalled.sh sets the necessary variables.
Signed-off-by: Simon McVittie <smcv@debian.org>