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 <mpratt@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
2 files changed