ci: only make use of necessary builtins

[vince-fugnitto]

Feature Description:

The project currently downloads/resolves the list of builtins during CI (for Linux) and tests against them during our API integration tests. The proposal is to not download and resolve the entire list of plugins (most notably the extension-pack) but instead only fetch the subset which are needed for the tests to pass.

The change would ultimately reduce build times, and put less unnecessary load on open-vsx.

A WIP which shows the tests pass with the subset can be viewed here (not the actual final implementation).

Any thoughts? @msujew @JonasHelming

[JonasHelming]

I do not see any drawbacks in the proposed solution, do I miss anything?

[msujew]

Sounds reasonable to me as well 👍