test: Configure Gitaly's bundler with env vars

Setting up and spawning Gitaly in tests needs to have some Bundler
settings configured. Until now this was done by writing to the local
config file in the ruby directory inside the gitaly dir used in test.

This method didn't seem to be robust for all our user and
use-cases (i.e. CI), because this configuration _could_ be done from
different places.

To make the situation more predictable, this change injects all Bundler
configuration through environment variables instead.