rustSetToolchainSource to use linux-x86 srcs

This was hardcoded in previously, and aosp/2746467 changed this to use
the HostPrebuiltTag instead which caused errors. Reverting back to
original implementation.

Change-Id: I8ed1919aa2ea737f7836fab39f9ee0663fb455d2
diff --git a/rust/toolchain_library.go b/rust/toolchain_library.go
index cb345a4..054104c 100644
--- a/rust/toolchain_library.go
+++ b/rust/toolchain_library.go
@@ -89,7 +89,7 @@
 
 func rustSetToolchainSource(ctx android.LoadHookContext) {
 	if toolchainLib, ok := ctx.Module().(*Module).compiler.(*toolchainLibraryDecorator); ok {
-		prefix := filepath.Join(config.HostPrebuiltTag(ctx.Config()), GetRustPrebuiltVersion(ctx))
+		prefix := filepath.Join("linux-x86", GetRustPrebuiltVersion(ctx))
 		versionedCrateRoot := path.Join(prefix, android.String(toolchainLib.Properties.Toolchain_crate_root))
 		versionedSrcs := make([]string, len(toolchainLib.Properties.Toolchain_srcs))
 		for i, src := range toolchainLib.Properties.Toolchain_srcs {