Fix EphemeralWorkspaceAdapter to accomodate plugin PVCs support

Commit dd47af60 changes the way that certain volumes are handled;
rather than storing the volume information in the relevant container's
machine, it is stored in the environment's pods directly. To accomodate
this, the EphemeralWorkspaceAdapter needs to pull the list of PVCs it
provisions from the environment rather than machines.

Signed-off-by: Angel Misevski <amisevsk@redhat.com>