[Builders] Dont rebuild gem index when unnecessary
[Builders] Dont rebuild gem index when unnecessary

This should speed up the specs slightly

RochesterinNYC/bundler,dilumnavanjana/bundler,mvz/bundler,mvz/bundler,dilumnavanjana/bundler,asutoshpalai/bundler,bundler/bundler,bundler/bundler,RochesterinNYC/bundler,asutoshpalai/bundler