gstpluginloader: when env var is set do not fall through to system plugin scanner

gstpluginloader: when env var is set do not fall through to system plugin scanner

If we set a custom GST_PLUGIN_SCANNER env var, then we probably want to use that and only that.

Falling through to the one installed on the system is problamatic in cross-compilation
environemnts, regardless of whether one pointed to by the env var succeeded or failed.

taken from:
http://cgit.openembedded.org/openembedded-core/commit/meta/recipes-multimedia/gstreamer/gstreamer1.0/0001-gst-gstpluginloader.c-when-env-var-is-set-do-not-fal.patch?id=0db7ba34ca41b107042306d13a6f0162885c123b

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/669>