daemon: Honor RPMOSTREE_USE_SESSION_BUS for startup too

While I was debugging the tests, I wanted to run the daemon
interactively under gdb but still on the session.  In our previous
logic this was only possible if we were exec'd by dbus-daemon which
made attaching gdb annoying.

Let's honor the environment variable consistently on client and
server.

src/daemon/main.c

@@ -361,6 +361,8 @@ main (int argc,
        * the system bus. */
       if (g_getenv ("DBUS_STARTER_BUS_TYPE") != NULL)
         bus_type = G_BUS_TYPE_STARTER;
+      else if (g_getenv ("RPMOSTREE_USE_SESSION_BUS") != NULL)
+        bus_type = G_BUS_TYPE_SESSION;
       else
         bus_type = G_BUS_TYPE_SYSTEM;