ci: make plugin download sequential

[vince-fugnitto]

What it does

The pull-request updates the download:plugins script to use sequential downloads as the new default (rather than parallel).

The changes include:

• the default for download:plugins is now set to parallel=false (sequential)
• the build step during CI is broken down into their respective parts (Install, Download Plugins and Build)
• we no longer unnecessarily call download;plugins for MacOS and Windows as we do not run integration tests there

How to test

Errors related to download:plugins (429) should occur less during CI.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

We should also respect this change in the gitpod config:

yarn --network-timeout 100000 && yarn build:examples

to

yarn --network-timeout 100000 && yarn build:examples && yarn download:plugins --parallel=false

[msujew]

Looks good to me 👍

I believe we should get this change into master before the release.