cmd/dist: correct gccheckmark test name

The test actually runs with gccheckmark=1, not gcstoptheworld=2.
Make the name match.

Change-Id: If38822a3f1ef65bc92fe47b375381df49a684c1d
Reviewed-on: https://go-review.googlesource.com/c/go/+/648755
Reviewed-by: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: cherrymui

src/cmd/dist/test.go

@@ -762,7 +762,7 @@ func (t *tester) registerTests() {
 			})
 		t.registerTest("GODEBUG=gccheckmark=1 runtime",
 			&goTest{
-				variant: "runtime:gcstoptheworld2",
+				variant: "runtime:gccheckmark",
 				timeout: 300 * time.Second,
 				short:   true,
 				env:     []string{"GODEBUG=gccheckmark=1"},