Have the ART Buildbot's build script build the Android Runtime APEX.

Enable it for target builds, so that the ART Buildbot exercises the
Runtime APEX module build rule.

Test: art/tools/buildbot-build.sh --target
Bug: 113373927
Change-Id: Iefe2a8ea4c0f4fc481e5f12afe9086eb9ed3f3bc
diff --git a/tools/buildbot-build.sh b/tools/buildbot-build.sh
index 3d70087..c1cc2e2 100755
--- a/tools/buildbot-build.sh
+++ b/tools/buildbot-build.sh
@@ -81,6 +81,8 @@
     # These targets are needed for the chroot environment.
     make_command+=" crash_dump event-log-tags"
   fi
+  # Build the Runtime APEX.
+  make_command+=" com.android.runtime"
   mode_suffix="-target"
 fi