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:

• [x] 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).
• [x] For content changes, I have completed the self-review checklist.

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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 :zap:

[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!