Switch from yarn to pnpm

We've started using pnpm throughout all Happo repos, and we find that
it's easier to use and faster.

Author: trotzig

.circleci/config.yml

@@ -9,5 +9,6 @@ jobs:
       - checkout:
           method: blobless
 
-      - run: yarn install
-      - run: yarn test
+      - run: npm install -g pnpm
+      - run: pnpm install
+      - run: pnpm test