Simplify PSPublishModule build import

[PrzemyslawKlys]

Summary

• replace the PSPublishModule feature-probing wrapper with a plain import
• remove environment-driven signing fallback logic from the build script
• keep signing scoped to the known signing machine, matching the existing module build pattern

Validation

• git diff --check
• parsed Build/Manage-PSWriteOffice.ps1 and Build/Validate-PackagedArtefact.ps1
• imported published PSPublishModule 3.0.5.2 and confirmed New-ConfigurationBuild exposes NETAssemblyLoadContext