docs: document WRANGLER_COMMAND environment variable for custom builds

[petebacondarwin]

Summary

Documents the new WRANGLER_COMMAND environment variable added to Wrangler's custom build commands in cloudflare/workers-sdk#12470.

Adds a new section to the Custom builds page covering:

• What WRANGLER_COMMAND is and when it's set
• Table of possible values (dev, deploy, versions upload, types)
• Example bash script showing how to branch build logic based on the variable

Things for reviewers to verify

• The table of WRANGLER_COMMAND values matches the implementation in workers-sdk#12470
• Whether the Configuration page's Custom builds section should also mention this env var (currently it just links to the custom-builds page)
• Whether a changelog entry is needed for this addition

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Link to Devin run: https://app.devin.ai/sessions/83e0c23d36b74427afe8df40388591db
Requested by: @petebacondarwin

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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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[devin-ai-integration]

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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/workers/wrangler/	@cloudflare/wrangler, @irvinebroque, @cloudflare/pcx-technical-writing, @cloudflare/product-owners

[github-actions]

Preview URL: https://f2a0f1b9.preview.developers.cloudflare.com
Preview Branch URL: https://devin-1770634867-wrangler-command-env-var.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/workers/wrangler/custom-builds/	https://devin-1770634867-wrangler-command-env-var.preview.developers.cloudflare.com/workers/wrangler/custom-builds/

[github-actions]

Hey there, we've marked this pull request as stale because there's no recent activity on it. This label is helps us identify PRs that might need updates (or to be closed out by our team if no longer relevant).

[github-actions]

Hey there, we've closed out this pull request because it's been stale for a while and there's been no additional action on it. If these changes are still relevant, open a new pull request (or flag to us in a GitHub issue).

[github-actions]

Hey there, we've marked this pull request as stale because there's no recent activity on it. This label helps us identify PRs that might need updates (or to be closed out by our team if no longer relevant).

[petebacondarwin]

Not stale - waiting for review

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.