bin/brew: quote in assignments for consistency
bin/brew: quote in assignments for consistency

While `var=$(<command>)` is perfectly valid and won't create any issues,
it is often surprising for less shell-savvy users. Quoting these cases
like in all other instances makes the code more uniform, helps to avoid
future mistakes, and has no downsides.

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