Update README with NuGet configuration instructions for Windows builds

[Copilot]

Summary

This PR updates the README with instructions for configuring NuGet credentials on Windows, addressing the issue raised in #17540 (comment).

Problem

Users attempting to build Cyberduck on Windows encounter 401/403 authentication errors when NuGet tries to access GitHub Package Registry sources (gh-ikvmnet and gh-iterate-ch). Without proper credentials configured, the build fails with cryptic error messages, blocking contributors from building the project locally.

Solution

Added a new "NuGet Configuration" section to the Windows prerequisites that provides clear, step-by-step instructions:

1. How to create a GitHub personal access token (classic) with read:packages permissions
2. Commands to configure NuGet sources with the credentials:

   dotnet nuget update source gh-ikvmnet -u "YourUsername" -p "YourPAT"
   dotnet nuget update source gh-iterate-ch -u "YourUsername" -p "YourPAT"

3. What values need to be replaced (GitHub username and PAT)

The section is placed logically in the Windows prerequisites, after "System Configuration" and before the "Building" section, ensuring users complete this setup before attempting to build.

Related

• Fixes authentication issues mentioned in Fix race conditions in FTP socket closure that cause intermittent errors #17540 (comment)
• References the official GitHub documentation for working with the NuGet registry

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Update build instructions in README with configuration for NuGet</issue_title>
<issue_description>As discussed in this comment ¹, the README for the project should be amended with build instructions for Windows that include properly setting credentials for nuget with a personal access token.

Comments on the Issue (you are @copilot in this section)

Fixes #17541

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Footnotes

1. https://github.com/iterate-ch/cyberduck/pull/17540#issuecomment-3361537789</issue_description> ↩

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[AliveDevil]

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This desperation.