sweet: disable get subcommand on plan9

Repo x/benchmarks tests have been failing on plan9 because the
sweet tool uses linux-dependent modules which don't build on plan9.
CL 689187 fixes the analogous problem for wasm, by making a
wasm-specific stub for the get subcommand. Here we extend the
change to apply to plan9 as well.

Fixes golang/go#74149

Change-Id: I25e232a5fbce001b74c63114577ac17d6bbf3bbe
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/693896
Auto-Submit: Michael Pratt <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: millerresearch

sweet/cmd/sweet/get_nowasm.go renamed to sweet/cmd/sweet/get_cipd.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
 

sweet/cmd/sweet/get_wasm.go renamed to sweet/cmd/sweet/get_nocipd.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.
 
+//go:build wasm || plan9
+
 package main
 
 import (
@@ -12,5 +14,5 @@ import (
 
 func (c *getCmd) Run(_ []string) error {
 	log.SetActivityLog(true)
-	return errors.New("get unsupported on wasm: CIPD not supported on wasm")
+	return errors.New("get unsupported on this platform: CIPD not supported on this platform")
 }