See this comment left by me for more context, specifically this part:
Also, we could install ShellCheck through apt-get on GitHub Actions to keep it consistent with Buildkite. To do this, we'd just have to revert the changes made in D4756.
On Buildkite, we install ShellCheck through apt-get, so this diff keeps Buildkite and GitHub Actions CI symmetric by installing ShellCheck through the same source.