Respect global packages folder in BuildToNuGetCache

[MrJul]

What does the pull request do?

This PR ensures that the configured NuGet global packages folder is respected in the BuildToNuGetCache build target, whether it's set via NuGet.config or the NUGET_PACKAGES environment variable.

What is the current behavior?

The path is hardcoded to the default ~/.nuget/packages folder.

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0048385-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[kekekeks]

I'm really reluctant to use any Nuke features at this point. They tend to break in spectacular ways with SDK updates

[MrJul]

It comes from the standard NuGet.Configuration NuGet package though, so hopefully that shouldn't break, as there's no Nuke wrapper involved here.

That being said, I went with the official API as it was the most convenient and compatible with NuGet.config overrides, but I'm only interested in the NUGET_PACKAGES environment variable if you prefer a hackish-way that doesn't use NuGet.Configuration.