utils: prefer keg curl over system on 10.6
utils: prefer keg curl over system on 10.6

Closes Homebrew/homebrew#39927.

Signed-off-by: Mike McQuaid <a17fed27eaa842282862ff7c1b9c8395a26ac320@mikemcquaid.com>

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