build: run jest with 7 workers in ci

[ashtonG]

Description

This limits the react tests to running with 7 workers in ci. This is because jest runs with one worker per detected cpu core and this number exceeds the cores we have available to us in the container. This can cause our test execution to get throttled, which can cause our tests to run slower than usual and possibly timeout. Because we're looking to move from jest anyways, the purpose of posting this pull request is to compare its runtimes to the runtimes on master and to the #5926 draft pr.

Test Plan

unit-test-react should continue to pass.

Checklist

• Changes have been manually QA'd
• User-facing API changes need the "User-facing API Change" label.
• Release notes should be added as a separate file under docs/release-notes/.
  See Release Note for details.
• Licenses should be included for new code which was copied and/or modified from any external code.

Ticket

[netlify]

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