cmd/dist: run cmd tests in race mode

[bcmills]

When race.bash was originally added (back in CL 7179052), the Go compiler was still written in C and thus was not meaningful to test under the race detector.
(I'm not sure why the rest of cmd was not tested under the race detector at that point. Perhaps @davecheney remembers?)

When race.bash was gutted and moved into cmd/dist, that behavior was retained:

go/src/cmd/dist/test.go

Lines 1529 to 1532 in b3b174f

	func (t *tester) shouldTestCmd() bool {
	if t.race {
	return false
	}

These days, cmd includes an extensive amount of Go code, much of which includes non-trivial concurrency, and some of which exercises packages that do not themselves have race-detector tests (#32783).

I think we should be testing all of cmd under the race detector, not just std. It will increase builder latency and potentially cost more resources, but the additional benefit of testing the concurrent code in cmd seems well worth the cost.

CC @golang/osp-team @josharian @dvyukov

[gopherbot]

Change https://golang.org/cl/224038 mentions this issue: cmd/dist: do not skip 'cmd' tests in race mode

[gopherbot]

Change https://golang.org/cl/225577 mentions this issue: cmd/go: do not append to the global cfg.OrigEnv slice