github_packages: Add platform.variant on Linux only
github_packages: Add platform.variant on Linux only

JCount/brew,JCount/brew,Homebrew/brew,sjackman/homebrew,MikeMcQuaid/brew,MikeMcQuaid/brew,EricFromCanada/brew,claui/brew,nandub/brew,claui/brew,vitorgalvao/brew,JCount/brew,Homebrew/brew,EricFromCanada/brew,vitorgalvao/brew,claui/brew,vitorgalvao/brew,nandub/brew,EricFromCanada/brew,MikeMcQuaid/brew,nandub/brew,claui/brew,Homebrew/brew,Homebrew/brew,sjackman/homebrew,JCount/brew,sjackman/homebrew,nandub/brew,MikeMcQuaid/brew,EricFromCanada/brew,vitorgalvao/brew,sjackman/homebrew