gst-uninstalled: include prefix in the uninstalled GST_PLUGIN_PATH

The uninstalled environment wires the PKG_CONFIG_PATH and LD_LIBRARY_PATH to correctly look into $GST/prefix. However, the same is not true for the GST_PLUGIN_PATH, so plugins in $GST/prefix/lib/gstreamer-1.0 will not be found.

I noticed that there is a $GST/prefix/plugins path in the current GST_PLUGIN_PATH, and I could use that, but it would imply a more fine-grained configuration, such as

./configure --prefix ~/gst/gst-1.14/prefix/ --plugindir=~/gst/gst-1.14/plugins

and not all projects provide this options. It would be more convenient if I could just set the --prefix accordingly.