Fix alpine linux RID graph generation

Fix alpine linux RID graph generation

Alpine Linux doesn't have 'linux' in its hierarchy, so the
GenerateRuntimeGraph target doesn't output alpine node to the runtimes
section of the Microsoft.NETCore.App.deps.json.

This change is a hotfix to that issue. In longer term, we should create
a new osgroup named `alpine` or maybe better `linuxmusl` or something
along these lines and make it a fully supported OS group everywhere.

Commit migrated from https://github.com/dotnet/core-setup/commit/16e103ca72e145a32a8ead2a4df6f325f943f53b