Waits for the manifests to be written before running the second loops

[arcanis]

What's the problem this PR addresses?

I forgot to add a Promise.all, so the package.json files could end up corrupted when the constraints engine had to perform multiple iterations of the loop.

Fixes #5853

How did you fix it?

Adds Promise.all before running subsequent iterations.

Checklist

• I have read the Contributing Guide.

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

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

[merceyz]

Do we have any tests that could possibly hit this? If we don't, could you add one?

[arcanis]

Since it involves parallel/sequential execution, I don't think I can write one that will fail 100% of the time.

[merceyz]

That's fine and also why I wrote "possibly hit this", a small chance of catching a regression is better than none at all.