cmd/dist: fix incorrect platform string shared by all tests

all tests currently share the same platform string and fail to
vet expected platforms

Fixes #19958

Change-Id: I2801e1e84958e31975769581e27ea5ca6a0edf5b
Reviewed-on: https://go-review.googlesource.com/40511
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/dist/test.go

@@ -340,7 +340,8 @@ var stdOutErrAreTerminals func() bool
 func (t *tester) registerTests() {
 	if strings.HasSuffix(os.Getenv("GO_BUILDER_NAME"), "-vetall") {
 		// Run vet over std and cmd and call it quits.
-		for osarch := range cgoEnabled {
+		for k := range cgoEnabled {
+			osarch := k
 			t.tests = append(t.tests, distTest{
 				name:    "vet/" + osarch,
 				heading: "go vet std cmd",