Add env variable HOMEBREW_BOTTLE_ARCH

Unless I have been mislead by my testing, setting the following env vars:
```
export HOMEBREW_BUILD_BOTTLE=1  # undocumented?
export HOMEBREW_BOTTLE_ARCH=core2  # new in this PR
```
would be equivalent to setting the following at command prompt:
```
brew install --build-bottle --bottle-arch=core2 <formula>
```
This would be *very* handy when using the new `Brewfile` support, e.g. set the env vars and issue `brew bundle`, thereby building, and possibly bottling, all/some of the formulae to spec, without having to be more verbose/arch-specific in the `Brewfile`.