Update customizing-the-containers-used-by-jobs.md

[mhuijgen]

The correct env var for the hooks is ACTIONS_RUNNER_CONTAINER_HOOKS See also https://github.com/actions/runner/blob/main/docs/adrs/1891-container-hooks.md Using ACTIONS_RUNNER_CONTAINER_HOOK does not work

Why:

Closes: #26156

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

Fix the env var name.

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Source	Preview	Production	What Changed
actions/hosting-your-own-runners/managing-self-hosted-runners/customizing-the-containers-used-by-jobs.md	fpt ghec	fpt ghec

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[cmwilson21]

@mhuijgen Thank you for opening a PR and linking it to your issue! ✨

We appreciate the extra context provided in the issue. I'll get this triaged for review! :zap:

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