Update CopyContentmentPackageAssets target to run BeforeBuild

[vsilvar]

Description

This ensure package consumers don't need to do a dotnet build before dotnet publish to have the package assets properly copied to the output

This issue was initially present on the Umbraco Package template, but has since been fixed in https://github.com/umbraco/Umbraco-CMS/pull/11992

Workaround

Currently package consumers have to either:

• Have separate build and publish steps
• Add the following target to their .csproj:

  <Target Name="EnsureCopyPackageAssets" BeforeTargets="BeforeBuild">
  <CallTarget Targets="CopyContentmentPackageAssets" />
  </Target>

This issue is also is also present in previous versions, so might be worth backporting the fix.

Steps to reproduce

Install the package Make sure the project is in a clean state and the assets aren't present in App_Plugins yet (dotnet clean) Run dotnet publish The output should now be missing the package assets

Types of changes

• [ ] Documentation change
• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [x] My code follows the coding style of this project.
• [x] My changes generate no new warnings.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the corresponding documentation.
• [x] I have read the CONTRIBUTING and CODE_OF_CONDUCT documents.