test-bot: cleanup Cellar/etc/var after each build.
test-bot: cleanup Cellar/etc/var after each build.

This has been causing issues when bottling files that are installed to
these paths due to the diff algorithm we use for figuring out whether
files installed into etc/var belong to a given bottle. Additionally,
sometimes not all formulae were uninstalled which meant `brew doctor`
would fail.

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