x/simulation: make signals channel buffered to avoid missing signals (#7952)

Invoking:

    c := make(chan os.Signal)
    signal.Notify(c, signals...)

is explicitly called out as a bug in the os/signal docs
and should instead make that channel buffered lest
a signal could be lost.

Fixes #7950

Co-authored-by: Aleksandr Bezobchuk <alexanderbez@users.noreply.github.com>

x/simulation/simulate.go

@@ -102,7 +102,7 @@ func SimulateFromSeed(
 	opCount := 0
 
 	// Setup code to catch SIGTERM's
-	c := make(chan os.Signal)
+	c := make(chan os.Signal, 1)
 	signal.Notify(c, os.Interrupt, syscall.SIGTERM, syscall.SIGINT)
 
 	go func() {