YAN-1221 Receive PVC name from env variable, not PB parameters

Historically we have received the name of the PVC used by the receiver via PB parameters. This however is changing in SP-2944: SDP now automatically creates a (or uses an existing) PVC, and communicates the name of this PVC down to scripts via an environment variable.

This commit changes the vis-receive script to read this environment variable to populate the required Helm values for the receive chart to pass down the information of the PVC name to be used by the receiver pod. To retain backwards compatibility, and as a fallback, we still support indicating the PVC name from the PB parameters.

NOTE: The receiver helm chart still lacks some intelligence regarding how it handles the PVC information. In particular, the value set by the vis-receive script is used to automatically mount the PVC in the receiver container, but not the others -- most notably, not in the processor container that does the final MS writing. This means that at the PB level we still need to provide this information manually. There is a tech debt ticket on our side to handle things more smoothly (https://jira.skatelescope.org/browse/YAN-1154). I've been working on that separately, but I'd rather not wait for those changes to get these in. Most likely the changes in the chart will be backwards-incompatible (I'm including not only the handling of the PVC, but simplifying many other details of the setup that at the moment are very awkward), so getting them in will require further discussion.