Explain that GITHUB_SHA may not always be the commit that was pushed

[danielcompton]

Why:

Closes: #15302

What's being changed:

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Writer impact (This section is for GitHub staff members only):

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  • I have added the 'writer impact' label
  • I have added a description and/or a video demo of the changes below (e.g. a "before and after video")

[ramyaparimi]

@danielcompton
Thanks so much for opening a PR! I'll get this triaged for review ⚡

[ethomson]

Thanks @danielcompton - this makes sense to me. 👍

[skedwards88]

Thanks for this addition! I left a minor wording suggestion that I will go ahead an incorporate, then we'll get this merged down for you.

[github-actions]

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The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

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Source	Preview	Production	What Changed
content/actions/learn-github-actions/environment-variables.md	Modified	Original

[github-actions]

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