Use shared format workflow

[delucis]

Description

This PR updates our format workflow to use the shared workflow from https://github.com/withastro/automation/ that we use in other repos.

While there, I also borrowed some details from how we run this elsewhere:

• Cancels in-progress formatting runs if another commit comes in
• Doesn’t run outside the withastro org (just in case someone is committing to main in a fork)

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Latest commit: 4deaa46

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size-limit report 📦

Path	Size
/index.html	5.61 KB (0%)
/_astro/*.js	26.03 KB (0%)
/_astro/*.css	16.05 KB (0%)