cmd/coordinator: fix trybots by adding linux-only build tag

Broken by CL 165779 where I forgot we have trybots in x/build again.

Change-Id: I92df000cbeefe9ec11375cf22e260692ac188848
Reviewed-on: https://go-review.googlesource.com/c/build/+/166077
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/cmd/coordinator/modproxy.go b/cmd/coordinator/modproxy.go
index 59cebbb..e840757 100644
--- a/cmd/coordinator/modproxy.go
+++ b/cmd/coordinator/modproxy.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build linux
+
 package main
 
 import (