ci: allow make unit_test_cover to run in parallel

[timvaillancourt]

Description

This PR allows make unit_test_cover to run in parallel, similar to other CI that rely on heavy go processing, example here: https://github.com/vitessio/vitess/blob/main/test/templates/unit_test.tpl#L137

It appears this override was never added to the code coverage CI. Let's see how it goes! This might address the bad flakiness we see

TIL: $(nproc) in GitHub Actions magically gives you the # of CPUs. We are already using this in other CI 👍

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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[vitess-bot]

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[timvaillancourt]

Update: this makes the code coverage CI run even slower. The likely cause is so many concurrent goroutines it makes the host slower than before

Closing this for now