feat: Added the --frozen-lockfile flag to the yarn_install command

BREAKING CHANGE: The `yarn_install` command will now fail if there need to be changes made to your `yarn.lock` file. See [the Yarn documentation](https://yarnpkg.com/en/docs/cli/install#toc-yarn-install-frozen-lockfile) for details on this flag.

Fixes #28

Author: matt-oakes

src/commands/yarn_install.yml

@@ -12,7 +12,7 @@ steps:
         - yarn-cache-{{ arch }}-{{ checksum "~/.tmp/checksumfiles/package.json" }}-{{ checksum "~/.tmp/checksumfiles/yarn.lock" }}-{{ .Environment.CACHE_VERSION }}
   - run:
       name: Yarn Install
-      command: yarn install --non-interactive --cache-folder ~/.cache/yarn
+      command: yarn install --frozen-lockfile --non-interactive --cache-folder ~/.cache/yarn
   - save_cache:
       paths:
         - ~/.cache/yarn