Add RUNNER_ENVIRONMENT to default env variables docs

[batbattur]

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Adds the RUNNER_ENVIRONMENT variable (same as runner.environment context) to the Default environment variables section.

Why:

The RUNNER_ENVIRONMENT is available but not documented anywhere.

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

Check off the following:

• [x] I have reviewed my changes in staging, available via the View deployment link in this PR's timeline (this link will be available after opening the PR).

  • For content changes, you will also see an automatically generated comment with links directly to pages you've modified. The comment won't appear if your PR only edits files in the data directory.

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

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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/learn-github-actions/variables.md	fpt ghec ghes@ 3.13 3.12 3.11 3.10	fpt ghec ghes@ 3.13 3.12 3.11 3.10

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

[nguyenalex836]

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

[github-actions[bot]]

Thanks for opening a pull request! We've triaged this issue for technical review by a subject matter expert :eyes:

[nguyenalex836]

@batbattur Thank you so much for your patience while our SME team reviewed your PR! The changes look great - I'll update the branch and get this merged once tests are passing 🍏

[github-actions[bot]]

Thanks very much for contributing! Your pull request has been merged 🎉 You should see your changes appear on the site in approximately 24 hours. If you're looking for your next contribution, check out our help wanted issues :zap: