The topobjdir-finding logic in mozbuild relies on MOZ_CURRENT_PROJECT
being set, and it's currently only set when going through client.mk.

On automation, during universal builds, make check is invoked directly
in one of the objdirs, so MOZ_CURRENT_PROJECT is not set. We've had
other similar problems in the past. Ensuring MOZ_CURRENT_PROJECT is set
in the objdir itself should reduce the risk of other such issues in the
future.