CI: Reduce log spam in CI to fix errors with Buildkite reporting

Created by: umpox

Description

We often get errors when trying to sending failing test logs to Buildkite.

Example failure: https://buildkite.com/sourcegraph/sourcegraph/builds/142872#ded1844b-649a-4b71-a944-fd42391f8f6c/2126-2130

This PR introduces two changes to reduce noise in CI logs:

• Adds --silent to yarn install so it will not print an installation log
  • I think this is an OK change as the chances of us being able to actually debug these logs are quite slim, and generally useful install logs are surfaced locally before happening in CI.
• Adds --silent to nyc so we do not print a code coverage report in CI.
  • This is mostly noise anyway. The report is still generated and uploaded to CodeCov - which is a much better place to view coverage.

Test plan

Tested in CI - with both failing and passing tests

App preview:

• Link
• Storybook

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