NuGet artifact shouldn't have dir structure

The original retained directory structure of after src/ and user thus
needed to copy all files from multiple directories 2 level deep.

Instead, pack all generated nuget packages to a single directory and
store them all in a root of zip archive.

.github/workflows/build.yml

@@ -52,13 +52,13 @@ jobs:
           files: tests/DataIsland.Core.Tests/TestResults/**/coverage.opencover.xml
 
       - name: Pack DataIsland.Core
-        run: dotnet pack --no-build --configuration Release /p:PackageVersion=1.0.0-${{ github.run_number }}-${{ github.sha }}
+        run: dotnet pack --no-build --configuration Release --output nuget_packages /p:PackageVersion=1.0.0-${{ github.run_number }}-${{ github.sha }}
 
       - name: Archive Core package
         uses: actions/upload-artifact@v4
         with:
           name: core-package
-          path: src/*/bin/Release/*.nupkg
+          path: nuget_packages/*.nupkg
 
       - name: Publish
         if: startsWith(github.ref, 'refs/tags/')