Always build mk2rbc/rbcrun

Currently, mk2rbc/rbcrun aren't built in certain phases
of the build, which causes the Starlark product configuration
to fail when using get_build_var if a regular build hasn't
been run before.

Bug: 226677850
Test: Manually
Change-Id: If85608eef63be25fddc4d916c82247c13eb0195f

soong_ui.bash

@@ -53,6 +53,8 @@ export TOP=$(gettop)
 source ${TOP}/build/soong/scripts/microfactory.bash
 
 soong_build_go soong_ui android/soong/cmd/soong_ui
+soong_build_go mk2rbc android/soong/mk2rbc/cmd
+soong_build_go rbcrun rbcrun/cmd
 
 cd ${TOP}
 exec "$(getoutdir)/soong_ui" "$@"