rust: set ANDROID_RUST_VERSION based on prebuilts

ANDROID_RUST_VERSION is always set to the default Rust version, rather
than than respecting the RUST_PREBUILTS_VERSION setting. This is not the
correct behavior, this CL resolves the issue.

Test: m nothing
Change-Id: I3f8a2fc3590b7313286278b77534bb669cc693d6
diff --git a/rust/builder.go b/rust/builder.go
index 00035b9..20ca5db 100644
--- a/rust/builder.go
+++ b/rust/builder.go
@@ -274,7 +274,7 @@
 		implicits = append(implicits, outputs.Paths()...)
 	}
 
-	envVars = append(envVars, "ANDROID_RUST_VERSION="+config.RustDefaultVersion)
+	envVars = append(envVars, "ANDROID_RUST_VERSION="+config.GetRustVersion(ctx))
 
 	if ctx.RustModule().compiler.CargoEnvCompat() {
 		if _, ok := ctx.RustModule().compiler.(*binaryDecorator); ok {