ci: Changed NUGET_SOURCE workflow to variable (#3364)

Signed-off-by: mrickard <maurice@mauricerickard.com>

Author: mrickard

.github/workflows/azure-site-extension.yml

@@ -107,4 +107,4 @@ jobs:
       - name: Publish site extension
         working-directory: cloud-tooling/azure-site-extension
         run: |
-          dotnet nuget push "${{ env.PACKAGE_FILENAME }}.nupkg" --api-key ${{ secrets.NUGET_API_KEY }} --source ${{ secrets.NUGET_SOURCE }}
+          dotnet nuget push "${{ env.PACKAGE_FILENAME }}.nupkg" --api-key ${{ secrets.NUGET_API_KEY }} --source ${{ vars.NUGET_SOURCE }}