plugin: fix build constraint to disable test on linux/arm64

CL 151478 was suppose to fix the build failure on linux/arm64 but the
build constraint didn't exclude linux/arm64 properly.

Fixes #28982

Change-Id: Ia80265b0adba0384cd28bc2deb1726418664975a
Reviewed-on: https://go-review.googlesource.com/c/151303
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

src/plugin/plugin_test.go

@@ -2,7 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !nacl !linux,arm64
+// +build !nacl
+// +build !linux linux,!arm64
 
 package plugin_test