(SDK-331) allow additional gems to be installed
(SDK-331) allow additional gems to be installed

* `bundle install --path` ignores all "system" installed gems and causes unnecessary package installs
* `bundle install` (without --path) installs into GEM_HOME, which by default is non-user writeable

To still use the pre-installed packages, but allow folks to install additional gems
this sets GEM_HOME to the user's cachedir, and put all other cache locations onto GEM_PATH

See https://stackoverflow.com/a/11277228 for background

This still requires that the exact required gem versions are available in
package_cachedir.

DavidS/pdk,DavidS/pdk,DavidS/pdk