Get our source first so ristretto.nix is available for checksum

.circleci/config.yml

@@ -46,6 +46,9 @@ jobs:
           command: |
             echo "export NIX_PATH=nixpkgs=https://github.com/NixOS/nixpkgs/archive/$NIXPKGS_REV.tar.gz" >> $BASH_ENV
 
+      # Get *our* source code.
+      - "checkout"
+
       - restore_cache:
           # Get all of Nix's state relating to the particular revision of
           # nixpkgs we're using.  It will always be the same.  CircleCI
@@ -77,9 +80,6 @@ jobs:
             - paymentserver-nix-store-v1-3c83ad6ac13b67101cc3e2e07781963a010c1624-
             - paymentserver-nix-store-v1-
 
-      # Get *our* source code.
-      - "checkout"
-
       - restore_cache:
           # Restore the cache of Stack's state.  This will have all of the
           # compiled Haskell libraries we depend on and even the compiled