WasmEdge plugins are loaded from the container's rootfs and not from the host

[jprendes]

Just bear in mind that the env-vars you are parsing are those in the container spec, and not those of the shim.
Also the plugin is loaded from the container's rootfs, and not from the host.
I'm happy to merge as is, and discuss if we want to switch to loading from the host in a different issue / PR.

Originally posted by @jprendes in #877 (review)