Gitaly should be recompiled when `GITALY_SERVER_VERSION` changes
Reported by @zj
The gitaly binary is installed prior to the tests being run, in order to support integration testing against Gitaly.
Currently, if the binary already exists it is never reinstalled. This can lead to a situation where the binary is out of date. This doesn't happen in CI since the environment is initialised on each run, but it can happen in developer's environments.