Fix Netlify builds for PRs

[12joan]

Description
The Netlify build workflows have been failing for some time. The error message Command failed with exit code 129: yarn build might indicate a problem with memory usage; if so, this forum thread indicates that limiting the number of CPUs during the build might address the problem.

Full error message

8:18:01 PM:    Generating static pages (0/27) ...
8:18:27 PM: ​
8:18:27 PM: "build.command" failed                                        
8:18:27 PM: ────────────────────────────────────────────────────────────────
8:18:27 PM: ​
8:18:27 PM:   Error message
8:18:27 PM:   Command failed with exit code 129: yarn build (https://ntl.fyi/exit-code-129)
8:18:27 PM: ​
8:18:27 PM:   Error location
8:18:27 PM:   In Build command from Netlify app:
8:18:27 PM:   yarn build
8:18:27 PM: ​
8:18:27 PM:   Resolved config
8:18:27 PM:   build:
8:18:27 PM:     command: yarn build
8:18:27 PM:     commandOrigin: ui
8:18:27 PM:     environment:
8:18:27 PM:       - NODE_VERSION
8:18:27 PM:       - REVIEW_ID
8:18:27 PM:     publish: /opt/build/repo/site/out
8:18:27 PM:     publishOrigin: ui
8:18:28 PM: Failed during stage 'building site': Build script returned non-zero exit code: 2 (https://ntl.fyi/exit-code-2)
8:18:28 PM: Build failed due to a user error: Build script returned non-zero exit code: 2
8:18:28 PM: Failing build: Failed to build site
8:18:28 PM: Finished processing build request in 1m43.368s

Context
Prior attempts: #5785 (addressing linter errors), #5809 (addressing stale browserlist). I'm not sure which of these potential issues is the root cause, or if multiple of them are. Setting the CPU limit fixed the problem. 🎉

[changeset-bot]

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Latest commit: 3dcf5fc

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[12joan]

Success! 4/4 of builds with the fix succeeded, and 1/1 of builds without the fix failed.

[ahoisl]

Would be great to get this PR in, because all other PRs are essentially stuck because of the netlify build issue 👏
I guess, limiting to a single CPU does not hurt...

[12joan]

@dylans has been merging PRs despite this issue, so they're not stuck per se, but it would definitely be nice to get the green checkmark back on mergeable PRs. Having a reliable preview build will also help with reviewing and testing.

The config file is a JS file, so it should be possible to apply the limit only in Netlify if anyone can figure out the right environment variable to check. That can probably happen as a separate PR, though.