Use a fixed path as prefix for GNOME runtime

[3v1n0]

Starting from next major runtime release, we should use a fixed path for the snap content such as /snap/extension/gnome-50/ so that all the libraries can be compiled with such prefix without relying on the env variables for all the kinds of redirections.

This should imply that all the snaps we build should use a bind layout.

So basically do the same that is already done for the GPU content snap:

https://github.com/canonical/snapcraft/blob/4ca52360505d64ebc9e4d2dd2a7153649e95a633/snapcraft/extensions/gnome.py#L143-L157