rust_bindgen: respect custom OUT_DIR

Bug: 172551033
Test: m
Change-Id: I1f760de7cb03ec0bcad5c66ac08bb88ca77c26cd
diff --git a/rust/bindgen.go b/rust/bindgen.go
index 3db2d65..09af649 100644
--- a/rust/bindgen.go
+++ b/rust/bindgen.go
@@ -32,7 +32,7 @@
 	bindgenClangVersion = "clang-r383902c"
 
 	//TODO(b/160803703) Use a prebuilt bindgen instead of the built bindgen.
-	_ = pctx.SourcePathVariable("bindgenCmd", "out/host/${config.HostPrebuiltTag}/bin/bindgen")
+	_ = pctx.HostBinToolVariable("bindgenCmd", "bindgen")
 	_ = pctx.SourcePathVariable("bindgenClang",
 		"${cc_config.ClangBase}/${config.HostPrebuiltTag}/"+bindgenClangVersion+"/bin/clang")
 	_ = pctx.SourcePathVariable("bindgenLibClang",