os/mac/xcode: add additional CLT download link.
os/mac/xcode: add additional CLT download link.

Sadly Apple don't seem to be providing these reliably.

Fixes #6796

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