Add shfmt automation

[dfarrell07]

Per a discussion during code review...

Nit: Should there be indentation inside this loop?

Which highlights that [shellcheck doesn't do format checking](https://github.com/koalaman/shellcheck/issues/2463). They recommend [shfmt](https://github.com/mvdan/sh). I can look into it if other people think it would be helpful.

Originally posted by @dfarrell07 in #1730 (comment)

[dfarrell07]

There's a GHA that combines shfmt and shellcheck (and another tool): https://github.com/marketplace/actions/sh-checker

Trade-off is that we'd lose the ability to run it locally, which I think is important.

[github-actions]

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