audit: ensure curl always has an HTTP url.
audit: ensure curl always has an HTTP url.

This makes using `HOMEBREW_CURL` significantly nicer as then the system
`curl` (that, on old versions, sucks at SSL) can download a version of
`curl` that it can build from source.

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