Ignore already installed brew packages, use relative build path
Ignore already installed brew packages, use relative build path

* `brew install` throws an error for already installed packages,
  therefore routing STDERR to /dev/null avoids too early termination
* The base directory is the chirp-repository, not the ci-folder,
  therefore `cd ../build` did not work, it has to be `cd build`
  instead

concretecloud/chirp,concretecloud/chirp,ganwell/chirp,ganwell/chirp,ganwell/chirp,concretecloud/chirp