Derive DOTNET_ROOT from dotnet's Microsoft.NETCore.App runtime listing and
export it to both explicit and inherited build environments. Preserve caller
overrides, use the build environment for discovery, and leave discovery
unchanged on probe failures instead of treating Homebrew's wrapper directory
as a runtime root.
Add regression coverage and document the packing failure and its resolution.
Based on PR #284.
Co-authored-by: Jose Rosendo <josemrb99@gmail.com>
