Use the cache to circumvent building

Since we use `stack` to run `hpack` to generate the cabal file,
we need `stack` to have its cache all setup properly.

We could eschew `hpack` to relax this requirement...

.circleci/config.yml

@@ -5,6 +5,12 @@ jobs:
       - image: haskell:8.2.2
     steps:
       - checkout
+      - restore_cache:
+          keys:
+            - v1-stack-{{ checksum "package.yaml" }}-{{ checksum "stack.yaml" }}
+            - v1-stack-{{ checksum "package.yaml" }}-
+            - v1-stack-
+          name: Restoring stack cache
       - run:
           name: Build sdist
           command: make sdist