Don't let xcrun find superenv tools.
Don't let xcrun find superenv tools.

Causes an infinite loop when trying to find e.g. missing gcc.

Closes Homebrew/homebrew#17426
References Homebrew/homebrew#17455

ilovezfs/brew,alyssais/brew,jmsundar/brew,aw1621107/brew,Linuxbrew/brew,amar-laksh/brew_sudo,pseudocody/brew,muellermartin/dist,maxim-belkin/brew,reitermarkus/brew,gregory-nisbet/brew,mgrimes/brew,palxex/brew,staticfloat/brew,JCount/brew,reitermarkus/brew,DomT4/brew,gordonmcshane/homebrew,nandub/brew,gregory-nisbet/brew,nandub/brew,AnastasiaSulyagina/brew,mgrimes/brew,Homebrew/brew,MikeMcQuaid/brew,alyssais/brew,jmsundar/brew,mahori/brew,AnastasiaSulyagina/brew,bfontaine/brew,konqui/brew,pseudocody/brew,zmwangx/brew,palxex/brew,maxim-belkin/brew,hanxue/linuxbrew,ilovezfs/brew,EricFromCanada/brew,JCount/brew,nandub/brew,MikeMcQuaid/brew,Linuxbrew/brew,EricFromCanada/brew,aw1621107/brew,Linuxbrew/brew,tonyg/homebrew,toonetown/homebrew,claui/brew,sjackman/homebrew,pseudocody/brew,gordonmcshane/homebrew,toonetown/homebrew,EricFromCanada/brew,claui/brew,konqui/brew,hanxue/linuxbrew,bfontaine/brew,mistydemeo/homebrew,toonetown/homebrew,mahori/brew,staticfloat/brew,DomT4/brew,sjackman/homebrew,Homebrew/brew,Homebrew/brew,konqui/brew,palxex/brew,reelsense/brew,MikeMcQuaid/brew,bfontaine/brew,muellermartin/dist,EricFromCanada/brew,Linuxbrew/brew,mahori/brew,JCount/brew,amar-laksh/brew_sudo,tonyg/homebrew,tonyg/homebrew,reelsense/brew,reitermarkus/brew,zmwangx/brew,maxim-belkin/brew,Homebrew/brew,aw1621107/brew,MikeMcQuaid/brew,hanxue/linuxbrew,amar-laksh/brew_sudo,vitorgalvao/brew,sjackman/homebrew,zmwangx/brew,staticfloat/brew,muellermartin/dist,nandub/brew,DomT4/brew,mgrimes/brew,vitorgalvao/brew,sjackman/homebrew,gordonmcshane/homebrew,rwhogg/brew,claui/brew,alyssais/brew,mahori/brew,DomT4/brew,vitorgalvao/brew,mistydemeo/homebrew,claui/brew,JCount/brew,mistydemeo/homebrew,reitermarkus/brew,ilovezfs/brew,gregory-nisbet/brew,reelsense/brew,konqui/brew,rwhogg/brew,AnastasiaSulyagina/brew,vitorgalvao/brew,rwhogg/brew,jmsundar/brew