Hotfix: create and pack empty file to add TFM dependency to Umbraco.Cms and Umbraco.Cms.Targets

[ronaldbarendse]

Prerequisites

• I have added steps to test this contribution in the description below

Description

PR #13475 added an MSBuild target that added an empty file in a framework specific folder to the Umbraco.Cms and Umbraco.Cms.Targets meta-packages, so they can only be installed into projects with a compatible TFM.

This worked fine if the package was created using dotnet pack, but the target wasn't executed when doing a separate dotnet build and dotnet pack --no-build (as done by the build pipeline), causing the file to not be included.

This fixes this problem and only executes the target when the NuGet package is actually created, so it also doesn't have to write the empty file when only doing a build (reducing disk I/O).

[ronaldbarendse]

With this in place, trying to install Umbraco.Cms (or Umbraco.Cms.Targets) version 11.0.0 into a project with an incompatible TFM (e.g. net6.0) will produce the following error: NU1202: Package Umbraco.Cms 11.0.0 is not compatible with net6.0 (.NETCoreApp,Version=v6.0). Package Umbraco.Cms 11.0.0 supports: net7.0 (.NETCoreApp,Version=v7.0).

Otherwise it will install successfully, but not pull any dependencies into the project (as those are .NET 7 specific) and probably result in some confusion as to why Umbraco isn't working.