x/simulation: unbuffered os.Signal channel passed into signal.Notify isn't correct

[odeke-em]

Noticed while auditing and testing code, as of tip at be10bcb, the code in x/simulation looks like this

cosmos-sdk/x/simulation/simulate.go

Lines 104 to 106 in be10bcb

	// Setup code to catch SIGTERM's
	c := make(chan os.Signal)
	signal.Notify(c, os.Interrupt, syscall.SIGTERM, syscall.SIGINT)

but that channel is unbuffered, which means that if signals are sent before the channel is read from, they'll be dropped.
The docs for os/signal.Notify at https://golang.org/pkg/os/signal/#Notify explicitly call this out

In Go1.17, I plan on producing a static analyzer that'll automatically flag these when go test runs, but my company will release it even earlier as per golang/go#9399.

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