desktop-launch should set GST_PLUGIN_PATH

[Michi Henning]

For the thumbnailer, we need to snap an additional gstreamer plugin (libav). The plugin goes into the usual location (usr/lib/<arch>/gstreamer-1.0) in our snap. However, the plugin isn't found unless we also set GST_PLUGIN_PATH in our code. It would be preferable for desktop-launch to set GST_PLUGIN_PATH to include our snap's usr/lib/<arch>/gstreamer-1.0 directory, so we wouldn't need to do the env var hackery ourselves once our process starts.

Note: I know that GST_PLUGIN_SYSTEM_PATH is set already, but that doesn't help us. We need GST_PLUGIN_PATH to be set as well.

[Michi Henning (michihenning)]

Setting this to invalid because bugs are being tracked in GitHub:

https://github.com/ubuntu/snapcraft-desktop-helpers/issues/49