Use sandbox from installer_context, force build_dir
Use sandbox from installer_context, force build_dir

If we create a new sandbox object here, `development_pods` value would
be nil because it's only saved when `store_local_path` methods is called.
This makes file_accessor to fail when looking for vendored_libraries for
local pods. Using sandbox in installer context fixes this issue.

Additionally, Rome assumes that build products will exist under
`Pods/../build` after build is completed, and it's default behavior of CP
due to setting of `SYMROOT` as `'${SRCROOT}/../build'`. This breaks
projects which provides a custom symroot during `post_install`.

I decided to override build dir for xcodebuild as building for Rome should
be kept separated from actual development lifecycle.

neonichu/Rome