brew: check system-provided curl and git on Linux
brew: check system-provided curl and git on Linux

Set HOMEBREW_FORCE_BREWED_CURL or HOMEBREW_FORCE_BREWED_GIT if the system-provided versions are missing or outdated, causing either to be installed on the next `brew update`.

Homebrew/brew,claui/brew,nandub/brew,mahori/brew,reitermarkus/brew,EricFromCanada/brew,JCount/brew,Linuxbrew/brew,nandub/brew,MikeMcQuaid/brew,reitermarkus/brew,sjackman/homebrew,claui/brew,Linuxbrew/brew,vitorgalvao/brew,sjackman/homebrew,JCount/brew,JCount/brew,konqui/brew,MikeMcQuaid/brew,claui/brew,reitermarkus/brew,mahori/brew,mahori/brew,vitorgalvao/brew,MikeMcQuaid/brew,nandub/brew,Homebrew/brew,sjackman/homebrew,nandub/brew,vitorgalvao/brew,EricFromCanada/brew,sjackman/homebrew,EricFromCanada/brew,reitermarkus/brew,JCount/brew,mahori/brew,Linuxbrew/brew,konqui/brew,claui/brew,Homebrew/brew,konqui/brew,konqui/brew,MikeMcQuaid/brew,vitorgalvao/brew,Linuxbrew/brew,Homebrew/brew,EricFromCanada/brew