C#: pass --skip-duplicate to dotnet nuget push (#7544)

NuGet versions are immutable, so the daily scheduled publish failed
whenever no new commit had landed since the previous run (the snapshot
version is derived from the latest commit timestamp). `--skip-duplicate`
turns the duplicate response into a logged warning instead of an error.

Author: timtebeek

rewrite-csharp/build.gradle.kts

@@ -313,7 +313,8 @@ val csharpPublish by tasks.registering(Exec::class) {
             findDotnet(), "nuget", "push",
             "dist/*.nupkg",
             "--source", "https://api.nuget.org/v3/index.json",
-            "--api-key", project.findProperty("nugetApiKey")?.toString() ?: ""
+            "--api-key", project.findProperty("nugetApiKey")?.toString() ?: "",
+            "--skip-duplicate"
         )
         logger.lifecycle("Publishing C# NuGet package (version: $nugetVersion)")
     }