Hardcode NuGet trusted publishing user

[jfversluis]

⚠️ Mandatory before the next release

This PR must be merged before the next release. Otherwise, NuGet trusted publishing will fail at the NuGet login (OIDC) step because the environment-scoped NUGET_USER secret is being removed.

Description of Change

Uses the literal nuget.org username jfversluis_MSFT for NuGet/login instead of reading it from secrets.NUGET_USER.

The username is a public publisher identifier, not a credential. Trusted publishing remains unchanged: GitHub Actions OIDC still obtains the short-lived NuGet API key exposed through steps.login.outputs.NUGET_API_KEY, and the workflow continues to use that key for package publishing.

Linked Issues

• N/A — focused release infrastructure follow-up

PR Checklist

• Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal) — N/A; release infrastructure fix
• Has tests (if omitted, state reason in description) — N/A; one-line workflow configuration change
• Has samples (if omitted, state reason in description) — N/A; no product behavior change
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Documentation created or updated: https://github.com/MicrosoftDocs/CommunityToolkit/pulls — N/A; no public API or documentation change

Additional information

Validation:

• Ruby YAML parser successfully parsed .github/workflows/dotnet-build.yml.
• git diff --check passed.
• The diff is limited to one insertion and one deletion in the NuGet/login user input; all OIDC permissions, action pinning, login output handling, and NuGet push behavior remain unchanged.

[Copilot]

Pull request overview

This PR updates the release pipeline to keep NuGet trusted publishing (OIDC) working after removal of the environment-scoped NUGET_USER secret, by switching the NuGet/login action’s user input to a literal nuget.org username.

Changes:

• Replaces ${{ secrets.NUGET_USER }} with the literal jfversluis_MSFT for NuGet/login’s user input.
• Removes the workflow’s dependency on the NUGET_USER secret while preserving OIDC-based API key acquisition and dotnet nuget push usage.

Show a summary per file

File	Description
.github/workflows/dotnet-build.yml	Hardcodes the NuGet publisher username for NuGet/login to avoid failure when NUGET_USER is removed.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Low