Export RustDefaultVersion to Bazel

The config constant is consumed by rust toolchain implemented in aosp/2709668.

Bug: 295918553
Test: go test
Change-Id: I0a28a802ad28f6ee24dfb1b5bdab72094534ae04
diff --git a/bp2build/conversion_test.go b/bp2build/conversion_test.go
index 15284ce..cbffaa0 100644
--- a/bp2build/conversion_test.go
+++ b/bp2build/conversion_test.go
@@ -117,6 +117,14 @@
 			basename: "constants.bzl",
 		},
 		{
+			dir:      "rust_toolchain",
+			basename: GeneratedBuildFileName,
+		},
+		{
+			dir:      "rust_toolchain",
+			basename: "constants.bzl",
+		},
+		{
 			dir:      "apex_toolchain",
 			basename: GeneratedBuildFileName,
 		},