Build: raise the build_wpcom timeout for trunk builds

[retrofox]

Proposed changes

Raise timeout-minutes on the build_wpcom job in .github/workflows/build.yml from 25 to 40, and refresh the stale comment next to it. Same change and same reason as #51029, which covered build_all but deliberately left the split jobs alone for lack of evidence. The evidence arrived a few hours after it merged.

Every trunk push builds the whole monorepo split into build_wpcom and build_nonwpcom (see the comment in the prepare job). Since #50509 landed, the wpcom half takes more than 25 minutes, so the job is cancelled at its cap on every trunk push: run 30993937046 died at 25m21s, and the follow-up run for the next trunk commit did the same. While that holds, Push wpcom plugins to mirror repos and Push to mirror repos never run and every mirror repo stays frozen at the last pre-#50509 state, which blocks the wpcomsh rolling releases and everything else that deploys from the mirrors.

build_nonwpcom finished in 14m53s on the same run, so its 25-minute value keeps a healthy margin and stays as is.

The mirror pipeline self-heals once this merges: trunk builds cover the full tree, so the first green run pushes all mirrors up to date.

Related product discussion/links

• Build: raise the timeout for full-monorepo builds #51029
• Limit wp-build esbuild concurrency WordPress/gutenberg#79889 (the upstream esbuild concurrency cap that gives the real headroom back)

Does this pull request change what data or activity we track or use?

No.

Testing instructions

Nothing to test on a site. Confirm the diff touches only the build_wpcom timeout in .github/workflows/build.yml. The value proves itself on the first trunk push after merge: Build wpcom projects should complete in roughly 26 to 30 minutes and the two mirror-push jobs should run.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack or WordPress.com Site Helper), and enable the fix/build-wpcom-timeout branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack fix/build-wpcom-timeout

bin/jetpack-downloader test jetpack-mu-wpcom-plugin fix/build-wpcom-timeout

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[louwie17]

LGTM 🚀 Already merged a similar update here: #51029, this one was missed, so 👍

[github-actions]

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