🔧(ci) use yarn.lock instead of package.json checksum as cache key

We were using a package.json checksum as cache key, but this is not
ideal as it means we still get cache hits if yarn.lock is updated
when package.json is not.

On the other hand, whenever package.json is updated, yarn.lock will
necessarily be updated too.

.circleci/config.yml

@@ -238,7 +238,7 @@ jobs:
       - checkout
       - restore_cache:
           keys:
-            - v3-front-dependencies-{{ checksum "package.json" }}
+            - v3-front-dependencies-{{ checksum "yarn.lock" }}
       - attach_workspace:
           at: ~/fun/src/richie/locale
       - run:
@@ -533,7 +533,7 @@ jobs:
       - save_cache:
           paths:
             - ./node_modules
-          key: v3-front-dependencies-{{ checksum "package.json" }}
+          key: v3-front-dependencies-{{ checksum "yarn.lock" }}
 
   lint-front-tslint:
     docker:
@@ -543,7 +543,7 @@ jobs:
       - checkout
       - restore_cache:
           keys:
-            - v3-front-dependencies-{{ checksum "package.json" }}
+            - v3-front-dependencies-{{ checksum "yarn.lock" }}
       - run:
           name: Lint code with tslint
           command: yarn lint
@@ -556,7 +556,7 @@ jobs:
       - checkout
       - restore_cache:
           keys:
-            - v3-front-dependencies-{{ checksum "package.json" }}
+            - v3-front-dependencies-{{ checksum "yarn.lock" }}
       - run:
           name: Lint JS/TS/JSON and CSS/SCSS code with prettier
           command: yarn prettier --list-different "src/richie-front/js/**/*.+(ts|tsx|json|js|jsx)" "*.+(ts|tsx|json|js|jsx)" "src/**/*.+(css|scss)"
@@ -569,7 +569,7 @@ jobs:
       - checkout
       - restore_cache:
           keys:
-            - v3-front-dependencies-{{ checksum "package.json" }}
+            - v3-front-dependencies-{{ checksum "yarn.lock" }}
       - run:
           name: Run tests
           # Circle CI needs the tests to be ran sequentially, otherwise it hangs. See Jest docs below: