bin/brew: don't set empty, unfiltered variables.
bin/brew: don't set empty, unfiltered variables.

Otherwise we unconditionally set e.g. `CI`, `TRAVIS`.

alyssais/brew,DomT4/brew,mgrimes/brew,reelsense/brew,reelsense/brew,ilovezfs/brew,DomT4/brew,JCount/brew,mgrimes/brew,mgrimes/brew,reitermarkus/brew,JCount/brew,mahori/brew,claui/brew,hanxue/linuxbrew,EricFromCanada/brew,konqui/brew,EricFromCanada/brew,DomT4/brew,MikeMcQuaid/brew,reitermarkus/brew,claui/brew,pseudocody/brew,sjackman/homebrew,Homebrew/brew,alyssais/brew,DomT4/brew,Linuxbrew/brew,maxim-belkin/brew,ilovezfs/brew,claui/brew,claui/brew,EricFromCanada/brew,mahori/brew,nandub/brew,mahori/brew,reelsense/brew,pseudocody/brew,konqui/brew,nandub/brew,vitorgalvao/brew,Linuxbrew/brew,konqui/brew,reitermarkus/brew,nandub/brew,sjackman/homebrew,konqui/brew,hanxue/linuxbrew,maxim-belkin/brew,vitorgalvao/brew,pseudocody/brew,Homebrew/brew,ilovezfs/brew,sjackman/homebrew,MikeMcQuaid/brew,nandub/brew,sjackman/homebrew,EricFromCanada/brew,mahori/brew,MikeMcQuaid/brew,vitorgalvao/brew,Homebrew/brew,MikeMcQuaid/brew,maxim-belkin/brew,reitermarkus/brew,Homebrew/brew,aw1621107/brew,JCount/brew,Linuxbrew/brew,vitorgalvao/brew,hanxue/linuxbrew,aw1621107/brew,aw1621107/brew,Linuxbrew/brew,JCount/brew,alyssais/brew