shims/scm: handle missing Subversion on Catalina.
shims/scm: handle missing Subversion on Catalina.

Fixes https://github.com/Homebrew/brew/issues/7781

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