make fetcher concurrency correlate with networkConcurrency

[goloveychuk]

What's the problem this PR addresses? Increasing yarn install concurrency, make it sort of configurable.

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How did you fix it? Using network concurrency. It could make sense to introduce new optional param (installConcurrency) and when it's not presented, fallback to networkConcurrenct+C.

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Checklist

• [x] I have read the Contributing Guide.

• [x] I have set the packages that need to be released for my changes to be effective.

• [x] I will check that all automated PR checks pass before the PR gets reviewed.