formula_installer: fix option-with-value handling
formula_installer: fix option-with-value handling

When passing formula options with value, e.g. `--with-qt=5`, to the
child process responsible for building a formula, `ARGV.value` would be
invoked with `nil`. Handle this more elegantly (no change in behavior).

For consistency, use a regular expression adapted from `Options.create`
instead of the somewhat bogus one used before.

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