GitHub action to publish .NET packages to NuGet

Using ${{env.config}} in dotnet pack - good catch, updated it.

Setting env.dotnet_core_version to 6.0.x is better as you have outlined. Note that it is still an environment variable and the value can be changed as you see it fit, it is just an example.

With regards to env.DOTNET_CLI_TELEMETRY_OPTOUT and env.DOTNET_SKIP_FIRST_TIME_EXPERIENCE, GH action will run fine but the first run of any dotnet command will eat up some time sending telemetry data to Microsoft so it is always better to set these flags to disable it.