Create a file in your mozilla-beta checkout called mozconfig. Then add the following to it, and then run ./mach build again.
ac_add_options --enable-artifact-builds
mk_add_options MOZ_OBJDIR=./objdir-frontend
ac_add_options --with-branding=browser/branding/aurora
Of course you can leave off the artifcat-builds line, if you want to compile from scratch and not using artifact builds.