cmd/goimports: fix -trace flag: flush after stopping trace

Without this change, the created trace files cannot be read by "go tool trace":

2023/12/25 09:51:02 Parsing trace...
failed to parse trace: no EvFrequency event

Change-Id: I59964875c8e821262db2cc6bbc82b2be385e1618
Reviewed-on: https://go-review.googlesource.com/c/tools/+/553555
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Michael Stapelberg <stapelberg@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

cmd/goimports/goimports_gc.go

@@ -19,8 +19,8 @@ func doTrace() func() {
 		bw, flush := bufferedFileWriter(*traceProfile)
 		trace.Start(bw)
 		return func() {
-			flush()
 			trace.Stop()
+			flush()
 		}
 	}
 	return func() {}