sdk: use unversioned SDK path on Big Sur
sdk: use unversioned SDK path on Big Sur

Fixes https://github.com/Homebrew/homebrew-core/issues/67075.

This implements @fxcoudert's suggestion in the issue linked above.

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