audit: do not warn about reachability of `brew mirror`ed URL
audit: do not warn about reachability of `brew mirror`ed URL

A `brew mirror`ed URL is usually not yet reachable at the time of pull request.

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