Upgrade to nixpkgs with upgraded PaymentServer build system

This was supposed to reduce evaluation-time memory enough to let low-memory
staging servers self-deploy.  But it didn't.

nixpkgs.json

 { "name": "nixpkgs"
-, "url": "https://github.com/PrivateStorageio/nixpkgs/archive/788cc5806d46b89013ddd59db589b748bc20435e.tar.gz"
-, "sha256": "1mjznn4i4524gl5aiapjpy2jzpac1fzp7jvnkamrh9090ndalhar"
+, "url": "https://github.com/PrivateStorageio/nixpkgs/archive/a5cbaadd9676e8c568061e92bbf5ad6a5d884ded.tar.gz"
+, "sha256": "0q5zknsp0qb25ag9zr9bw1ap7pb3f76bxsw81ahxkzj4z5dw6k2f"
 }