Fix tests with cc_binary_host on mac

cc_binary_host on mac uses ctx.Config().HostSystemTool(), which needs
PATH in the test environment.  Copy it from the original environment.

Fixes: 129763458
Test: proto_test.go
Change-Id: I41c1acdceee7c35036148256adafb471871034df

cc/proto_test.go

@@ -15,7 +15,6 @@
 package cc
 
 import (
-	"runtime"
 	"strings"
 	"testing"
 
@@ -38,9 +37,6 @@ func TestProto(t *testing.T) {
 	})
 
 	t.Run("plugin", func(t *testing.T) {
-		if runtime.GOOS != "linux" {
-			t.Skip("TODO(b/129763458): cc_binary_host tests fail on mac when trying to exec xcrun")
-		}
 		ctx := testCc(t, `
 		cc_binary_host {
 			name: "protoc-gen-foobar",