Use empty string for core image variant

Use the empty string for the core image variant so that modules
added to imageMutator do not change their build directory.

Bug: 142286466
Test: m checkbuild
Change-Id: Ida4534d9a4d6176236aaa480fed359ce27acfaa1
diff --git a/rust/rust_test.go b/rust/rust_test.go
index 91c2f09..01b89db 100644
--- a/rust/rust_test.go
+++ b/rust/rust_test.go
@@ -245,7 +245,7 @@
 			srcs: ["foo.rs"],
                         no_stdlibs: true,
 		}`)
-	module := ctx.ModuleForTests("fizz-buzz", "android_arm64_armv8-a_core").Module().(*Module)
+	module := ctx.ModuleForTests("fizz-buzz", "android_arm64_armv8-a").Module().(*Module)
 
 	if android.InList("libstd", module.Properties.AndroidMkDylibs) {
 		t.Errorf("no_stdlibs did not suppress dependency on libstd")