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
diff --git a/tests/lib.sh b/tests/lib.sh
index 55f9ab4..1bb2df9 100644
--- a/tests/lib.sh
+++ b/tests/lib.sh
@@ -83,12 +83,14 @@
 function create_mock_soong {
   copy_directory build/blueprint
   copy_directory build/soong
+  copy_directory build/make/tools/rbcrun
 
   symlink_directory prebuilts/go
   symlink_directory prebuilts/build-tools
   symlink_directory prebuilts/clang/host
   symlink_directory external/go-cmp
   symlink_directory external/golang-protobuf
+  symlink_directory external/starlark-go
 
   touch "$MOCK_TOP/Android.bp"
 }