Update container hook environment variable name

[david-smith-cko]

Why:

The environment variable name used for specifying the path to a container hook script was incorrect. This was cross referenced against the action runner's source code: https://github.com/actions/runner/blob/main/src/Runner.Common/Constants.cs#L208

What's being changed (if available, include any code snippets, screenshots, or gifs):

The name of the environment variable has been updated in the docs.

Check off the following:

• I have reviewed my changes in staging (look for the "Automatically generated comment" and click the links in the "Preview" column to view your latest changes).
• For content changes, I have completed the self-review checklist.

[welcome]

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[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
actions/hosting-your-own-runners/customizing-the-containers-used-by-jobs.md	fpt ghec	fpt ghec

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server
ghae: GitHub AE

[cmwilson21]

@david-smith-cko Thanks so much for opening a PR! I'll get this triaged for review ⚡

[SiaraMist]

Hey @david-smith-cko, thank you so much for making this fix and for your patience as we work through our backlog. 💛 It looks like someone else spotted this and updated it too, so I'm going to go ahead and close this PR. We appreciate your help improving the docs!