cmd/dist: run racebench tests only in longtest mode
The racebench tests represent a significant portion of the race
builders' runtimes, but these tests aren't providing a lot of value.
Disable them and run them only on -longtest-race builders.
For #32032.
Change-Id: Ic4383c3f3b51d123ae9f5c377bc0e2ec02f2ddd7
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-race,gotip-linux-amd64-longtest-race
Reviewed-on: https://go-review.googlesource.com/c/go/+/700455 Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Bypass: Michael Knyszek <mknyszek@google.com>