Simplify build_package_rbx

This removes the `isolated_gem_dependency` hack as well as the `rake` &
`bundle` shell wrapper functions. The main reason is to avoid having to
use the `command` shell builtin to be able to invoke the real `rake` and
`bundle`. In my testing on bash 3.2, `command` does not respect the
usual bash error handling rules, and thus a failed `command` invocation
can trigger the ERR trap mechanism even when we don't want it to.