vendor-install: check -x on output of which
vendor-install: check -x on output of which

This works around older versions of `which`, which return a
string on failure.

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