Switch from npm to yarn for consistency

We're using yarn workspaces for snapshot tests

Author: olafurpg

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - run: npm install
+      - run: yarn install
       - run: yarn install
         working-directory: snapshots
       - run: npm run test