brew-unpack: use extend instead of reopening Formula
brew-unpack: separate args passed to system
brew-unpack: don't chdir further than a normal install would

If we want to generate usable patches from inside the unpacked source,
then we should start from the same directory that a normal install
would start from.
gist 4.2.1
yaz: add test
exiftool 9.54
translate-toolkit: let scripts find python modules
Add ARGV.env
Don't set installer options to their default

ignore_deps and show_header default to false.
Whitelist arguments that are passed to build process
Remove unnecessary check for pour_bottle?

@start_time will be nil if we're pouring a bottle, so we don't need to
also check pour_bottle?.
Add predicate methods for inspecting the installer mode
Make interactive an installer mode
Make verbose an installer mode
Make debug an installer mode
Make FormulaInstaller setup more readable
Simplify ARGV.filter_for_dependencies