sweet/benchmarks/etcd: don't build on Plan 9

This package has no relevance for Plan 9 and won't build because of
references to undefined symbols like syscall.UnixRights. Add a
!plan9 build tag to avoid this.

Fixes golang/go#68287

Change-Id: I4a0bb76d26f585b8e0371febb98ae0c57b84c237
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/596437
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>

sweet/benchmarks/etcd/main.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !wasm
+//go:build !wasm && !plan9
 
 package main