cmd/go: fix unbuffered channel passed to signal.Notify

Unbuffered channels passed into signal.Notify can be lost
as the docs for signal.Notify caution with:

    Package signal will not block sending to c: the caller must ensure
    that c has sufficient buffer space to keep up with the expected signal
    rate. For a channel used for notification of just one signal value,
    a buffer of size 1 is sufficient.

Found by a static analyzer from Orijtech, Inc. called "sigchanyzer", but
it'll be donated to the Go project soon.

Updates #9399.

Change-Id: Ia0690e447582da028694ed65ace7b97961997b84
Reviewed-on: https://go-review.googlesource.com/c/go/+/274332


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
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Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
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