Only enable rust_toolchain_filegroup targets on Linux hosts

The Rust toolchain prebuilts only include stdlib sources on Linux hosts.

Test: m nothing
Bug: 384401387
Change-Id: I81cd2dfe6dcd0420572fbb6c520a4ffc199b2965
diff --git a/soong/toolchain_filegroup.go b/soong/toolchain_filegroup.go
index ef4630f..bcfcc1d 100644
--- a/soong/toolchain_filegroup.go
+++ b/soong/toolchain_filegroup.go
@@ -18,10 +18,13 @@
 
 import (
 	"path"
+	"runtime"
 
 	"android/soong/android"
 	"android/soong/rust"
 	"android/soong/rust/config"
+
+	"github.com/google/blueprint/proptools"
 )
 
 // This module contains logic for defining file groups inside of a Rust
@@ -51,10 +54,17 @@
 
 func rustSetToolchainFileGroupSource(ctx android.LoadHookContext, inProps *toolchainFilegroupProps) {
 	type props struct {
-		Srcs []string
+		Srcs    []string
+		Enabled *bool
 	}
 
 	p := &props{}
+	if runtime.GOOS == "linux" {
+		p.Enabled = proptools.BoolPtr(true)
+	} else {
+		p.Enabled = proptools.BoolPtr(false)
+	}
+
 	prebuiltsDir := path.Join(config.HostPrebuiltTag(ctx.Config()), rust.GetRustPrebuiltVersion(ctx))
 	for _, src := range inProps.Toolchain_srcs {
 		p.Srcs = append(p.Srcs, path.Join(prebuiltsDir, src))