Hardcoded paths for pipewire and gstreamer

[soumyaDghosh]

With the paths of pipewire and gstreamer being hardcoded, it's not easy to change the paths without using a launcher script. Instead of hardcoding, why can't we add them in the snapcraft extensions part, where those env variables will be added in the apps.env part, like here,

https://github.com/snapcore/snapcraft/blob/main/snapcraft/extensions/gnome.py#L147