Build: Fix parallelism count

[yannbf]

Closes #

What I did

Apart from fixing the count, I improved the yarn get-template script so when it fails, it provides a proper answer that guides us with exactly what parallelism values we need put to fix the issue:

You can also run the command locally before pushing:

yarn get-template --cadence merged --task test-runner --debug

If you specify a task but not a cadence, it shows that task for all cadences:

If you specify a cadence, but not a task, it shows all tasks for that given cadence:

If you do not specify anything, it shows every cadence and every task:

A future iteration could, instead of logging the information, just automatically update the yml file instead.

How to test

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]