Problem with different installation path in Mac

https://github.com/game-ci/unity-builder/blob/3a2abf9037d0df5648e9b888402654639215fa1e/dist/platforms/mac/steps/activate.sh#L17

This assumes that the Unity installation will always be in `/Applications` , but the new Unity Hub allows for different custom installation paths, so this hard installation path creates problems for people with, for example, Unity installations on secondary drives.