[CI] Add “Install ShellCheck” step to ShellCheck GitHub Actions workflow

Summary:
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.

Test Plan: See what happens on GitHub Actions.

Reviewers: atul, tomek

Reviewed By: atul

Subscribers: ashoat, adrian

Differential Revision: https://phab.comm.dev/D4828