Documents environment variables present in workers builds

[Maximo-Guk]

Summary

Addresses part of #22809

Documents environment variables present in workers builds

Screenshots (optional)

Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

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PR Change Summary

Documented environment variables used in Workers builds to enhance clarity and usability for developers.

• Added a section on environment variables in Workers builds documentation.
• Outlined steps to access and configure environment variables in the Cloudflare dashboard.
• Listed default system environment variables with their injected values and use cases.

Modified Files

• src/content/docs/workers/ci-cd/builds/configuration.mdx

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[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/docs/pages/	@cloudflare/workers-docs, @GregBrimble, @WalshyDev, @aninibread, @irvinebroque, @cloudflare/pcx-technical-writing, @kodster28
/src/content/docs/workers/ci-cd/	@irvinebroque, @aninibread, @GregBrimble, @kodster28, @cloudflare/pcx-technical-writing

[github-actions]

Preview URL: https://ed98aab2.preview.developers.cloudflare.com
Preview Branch URL: https://maximo-add-workers-build-env-vars.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/workers/ci-cd/builds/configuration/	https://maximo-add-workers-build-env-vars.preview.developers.cloudflare.com/workers/ci-cd/builds/configuration/
https://developers.cloudflare.com/pages/configuration/build-configuration/	https://maximo-add-workers-build-env-vars.preview.developers.cloudflare.com/pages/configuration/build-configuration/