[CI] Add ShellCheck workflow to Buildkite

Summary:
Relevant Linear issue here.

This diff adds ShellCheck to the CI, a static analysis tool for shell scripts that provides warnings and errors.

We install ShellCheck and also set globstar so we can use ** to recursively match all .sh files in the repository. These files are then evaluated by ShellCheck.

Thank you to @atul for your help on this!

Test Plan: Ran in Buildkite and worked as expected. Currently the build is failing since we have so many shell scripts that contain problematic code, but that's expected! You can run it here on Buildkite as well.

Reviewers: atul, palys-swm, def-au1t, karol-bisztyga, varun

Reviewed By: atul

Subscribers: ashoat, Adrian

Differential Revision: https://phab.comm.dev/D4485