formula_installer: fix --default-fortran-flags
formula_installer: fix --default-fortran-flags

Setting the `FC` environment variable allows the user to override the
default Fortran compiler. The `SharedEnvExtension#fortran` method then
checks whether any of `FCFLAGS` and `FFLAGS` environment variables are
set and, if not, suggests to use `--default-fortran-flags` when running
`brew install`. However, `--default-fortran-flags` never makes it to
`build.rb` (that is executed by `FormulaInstaller`) because the flag is
not whitelisted in `FormulaInstaller#sanitized_ARGV_options`.

This change rectifies this and thereby eliminates the confusing message.

Fixes Homebrew/homebrew#47852.

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