[release tool]: run nvm gracefully to ensure caretaker uses correct node version for publish branch

The merge tool will currently check out other branches, such as patch, LTS etc. For that the node versions could be different and breaking. This currently results in some inconvenient Yarn install engines error mid-publishing in a way where recovering is rather cumbersome..

We should run `nvm` automatically, gracefully as not everbody might have it. If it fails, we print a warning and proceed running the Yarn install. That one could still fail in such cases, but this is expected then.

Note: We have considered using the Bazel-managed Node version (that would be ideal).. but we cannot let Bazel install the node modules as Bazel itself is actually made available through the `@bazel/bazelisk` package (caretaker might not have the proper Bazel version globally installed; if at all)

merge tool の branch-checkout と Yarn install のフローから着手し、特に publishing が patch または LTS ブランチに切り替わる箇所を確認してください。現在ツールが環境セットアップをどのように処理しているかを確認し、そのうえで nvm の試行が適切に処理され、利用できない場合や失敗した場合に警告が出力され、Yarn install が引き続き実行されることを検証してください。

索引モデルが issue の本文から書いたものです。