list: exclude zsh completions from unbrewed
list: exclude zsh completions from unbrewed

Since edf000e4cd30c3626ccc28c52ed32f2d84a200dd `zsh` completions are in
`share/zsh/site-functions/_brew`, making this path a part of Homebrew.
Hide it from `brew list --unbrewed` to reduce confusion/noise.

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