message: ci: cache Material `node_modules` based on the lockfile checksum
  This will increase the cache hit rate for the `material-unit-tests` job.
  Related to
  PR Close
diff: @@ -573,15 +573,14 @@ jobs:
  steps:
  - *attach_workspace
  - *init_environment
  - - run:
  - name: "Determining SHA of Material Ivy branch"
  - command: git ls-remote ${MATERIAL_REPO_URL} ${MATERIAL_REPO_BRANCH} | cut -f1 > material_repo_sha
  - run:
  name: "Cloning Material repository"
  command: ./scripts/ci/clone_angular_material_repo.sh
  - restore_cache:
  + # Material directory must be kept in sync with the `$MATERIAL_REPO_TMP_DIR` env variable.
  + # It needs to be hardcoded here, because env variables interpolation is not supported.
  keys:
  - - v2-angular-material-{{ checksum "material_repo_sha" }}
  + - v2-angular-material-{{ checksum "/tmp/material2/yarn.lock" }}
  - v2-angular-material-
  - run:
  name: Installing Material dependencies.
  @@ -590,9 +589,10 @@ jobs:
  # because we don't want to cache the node modules which have been modified to contain
  # the attached Ivy package output.
  - save_cache:
  - key: v2-angular-material-{{ checksum "material_repo_sha" }}
  + # Material directory must be kept in sync with the `$MATERIAL_REPO_TMP_DIR` env variable.
  + # It needs to be hardcoded here, because env variables interpolation is not supported.
  + key: v2-angular-material-{{ checksum "/tmp/material2/yarn.lock" }}
  paths:
  - # Needs to be hardcoded because environment variables are not interpolated here.
  - "/tmp/material2/node_modules"
  - run:
  name: "Running Material unit tests"
  