Bump zkap-spending-service to a version that supports nixos 21.11.

nixos/pkgs/zkap-spending-service/repo.json

-{
-  "owner": "privatestorage",
-  "repo": "zkap-spending-service",
-  "rev": "cbf7509f429ffd6e6cf37a73e4ff84a9c5ce1141",
-  "branch": "main",
-  "domain": "whetstone.privatestorage.io",
-  "outputHash": "04g7pcykc2525cg3z7wg5834s7vqn82xaqjvf52l6dnxv3mb9xr93kk505dvxcwhgfbqpim5i479s9kqd8gi7q3lq5wn5fq7rf7lkrj",
-  "outputHashAlgo": "sha512"
-}
+{
+  "owner": "privatestorage",
+  "repo": "zkap-spending-service",
+  "rev": "66fd395268b466d4c7bb0a740fb758a5acccd1c4",
+  "branch": "main",
+  "domain": "whetstone.privatestorage.io",
+  "outputHash": "1nryvsccncrka25kzrwqkif4x68ib0cs2vbw1ngfmzw86gjgqx01a7acgspmrpfs62p4q8zw0f2ynl8jr3ygyypjrl8v7w8g49y0y0y",
+  "outputHashAlgo": "sha512"
+}