Fix Foolscap protocol issues.

It fails

Update the deployment to the now-current version of our various bits of software.

This fixes build issues relating to OpenSSL 1.0.2 as well as adding a basic level of
PrivacyPass-based functionality.

nixpkgs Python tools changed to no longer set PYTHONHOME for us
automatically.

overrides

They're not necessary on newly selected nixpkgs revision.

The layer above will have to make sure that it is.

Bump to ZKAPAuthorizer that fixes the Ristretto thing.

This gives us the merged version of Ristretto support.

Add Ristretto to the end-to-end system test.