cmd/dist: skip API check on plan9 builders

The plan9-arm builder has a very slow filesystem and frequently times
out on this test. The api check verifies the API for all supported
GOOS/GOARCH/CGO_ENABLED combination anyway, so if we skip it on one
builder (or even most builders) there should be no loss of coverage.

Updates #37951

Change-Id: I86a93df2ec60a6af6d942e3954eef09ce67bb39e
Reviewed-on: https://go-review.googlesource.com/c/go/+/225662


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>