rust: Don't produce coverage variants for stublibs

Stubs don't have code by definition, so we shouldn't be producing
coverage variants for them.

Bug: 393556022
Test: m <cov stubs> # inidicates target no longer exists
Change-Id: I82a2848075259b9f115b19c46019ada3d29bad41
diff --git a/rust/library.go b/rust/library.go
index 95e7099..3686bf9 100644
--- a/rust/library.go
+++ b/rust/library.go
@@ -202,6 +202,9 @@
 }
 
 func (library *libraryDecorator) nativeCoverage() bool {
+	if library.BuildStubs() {
+		return false
+	}
 	return true
 }