roachtest: only use timestamp when taking snapshot for invariant problem

We take the risk that two tests could attempt to create a snapshot
at the same exact millisecond, as we have a 63 character limit on
the snapshot name and the cluster name usually exceeds this by itself.

Author: DarrylWong

pkg/cmd/roachtest/test_runner.go

@@ -1834,8 +1834,11 @@ func (r *testRunner) maybeSaveClusterDueToInvariantProblems(
 	for _, det := range dets {
 		if det.Stdout != "" {
 			_ = c.Extend(ctx, 7*24*time.Hour, t.L())
-			timestamp := timeutil.Now().Format("20060102_150405")
-			snapName := fmt.Sprintf("invariant-problem-%s-%s", c.Name(), timestamp)
+			timestamp := timeutil.Now().UnixMilli()
+			// We take the risk that two tests could attempt to create a snapshot
+			// at the same exact millisecond, as we have a 63 character limit on
+			// the name and the cluster name usually exceeds this by itself.
+			snapName := fmt.Sprintf("invariant-problem-%d", timestamp)
 			if _, err := c.CreateSnapshot(ctx, snapName); err != nil {
 				t.L().Printf("failed to create snapshot %q: %s", snapName, err)
 				snapName = "<failed>"