add rust_toolchain_rustc_prebuilt module type

Sandboxing the rust rules requires having explicit inputs for the rust
toolchain. This module type makes the rustc prebuilt and its
dependencies explicit as a module that can be depended on by all rust
rules.

Bug: 286077158
Test: rust sandboxing topic + go test
Change-Id: If2b80b32e329e6a6ee11ba824de868cf04714553
diff --git a/cc/testing.go b/cc/testing.go
index d1632aa..dbdee9e 100644
--- a/cc/testing.go
+++ b/cc/testing.go
@@ -35,6 +35,7 @@
 
 	multitree.RegisterApiImportsModule(ctx)
 
+	ctx.RegisterModuleType("prebuilt_build_tool", android.NewPrebuiltBuildTool)
 	ctx.RegisterModuleType("cc_benchmark", BenchmarkFactory)
 	ctx.RegisterModuleType("cc_object", ObjectFactory)
 	ctx.RegisterModuleType("cc_genrule", GenRuleFactory)