ci: fix pipeline failing on duplicate NuGet package push

[Copilot]

Re-runs of the CI workflow on the same commit/tag produce the same effective NuGet version because NuGet ignores SemVer build metadata (+N) when comparing versions, causing a 409 Conflict on the second push.

Changes

• --skip-duplicate added to dotnet nuget push in ci.yml — silently skips already-published versions instead of failing
• MINVERBUILDMETADATA syntax fixed — changed build.$GITHUB_RUN_ATTEMPT (unexpanded shell variable reference) to build.${{ github.run_attempt }} (valid GitHub Actions expression)

# Before
MINVERBUILDMETADATA: build.$GITHUB_RUN_ATTEMPT
run: dotnet nuget push "./artifacts/*.nupkg" --source "github" --api-key ${{ secrets.GITHUB_TOKEN }}

# After
MINVERBUILDMETADATA: build.${{ github.run_attempt }}
run: dotnet nuget push "./artifacts/*.nupkg" --source "github" --api-key ${{ secrets.GITHUB_TOKEN }} --skip-duplicate

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