Use the new CI feed to download the pre-built artifacts

[mattleibow]

This pull request updates the NuGet package feed URL used for downloading and comparing packages from the preview feed to the CI artifacts feed. The change ensures that the build and utility scripts now reference the CI feed instead of the previous preview feed.

Feed URL update:

• Updated the variable from PREVIEW_FEED_URL to CI_ARTIFACTS_FEED_URL in build.cake and changed its default value to point to the SkiaSharp CI artifacts NuGet feed.
• Updated all references in scripts/cake/UtilsManaged.cake to use CI_ARTIFACTS_FEED_URL instead of PREVIEW_FEED_URL for package source and comparison operations.

[github-actions]

Triage Summary

Labels will be applied to indicate the affected backend (SkiaSharp) and the specific area of the issue (Build process).

This issue is not a regression, as it focuses on updates to the NuGet package feed without affecting compatibility with previous versions.

Additional remarks:

• The issue mainly addresses the process of updating the NuGet package feed URL and references in build scripts. There are no concerns related to compatibility or performance, and no operating system-specific labels were applicable.

Detailed Summary and Actions

Summary of the triage:

• The issue involves updates to the NuGet package feed for the SkiaSharp CI artifacts.
• It pertains to the Build process, specifically regarding the adjustment of build scripts to reference CI artifacts.
• There are no regression concerns, as it does not relate to previous version compatibility.

Summary of the actions that will be performed:

Action	Item	Description
Apply Label	backend/SkiaSharp	The issue specifically mentions updating the NuGet package feed for SkiaSharp CI artifacts.
Apply Label	area/Build	The issue pertains to changes in the build process related to the NuGet package feed references.

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