execext.RunCommand: fix: do not pass a cancellable context to mvdan.cc/sh
execext.RunCommand: fix: do not pass a cancellable context to mvdan.cc/sh

We used to pass to mvdan.cc/sh/interp.Runner a context that was cancelled on
reception of a OS signal. This caused the Runner to terminate the subprocess
abruptly.

The correct behavior instead is for us to completely ignore the signal and let
the subprocess deal with it. If the subprocess doesn't handle the signal, it
will be terminated. If the subprocess does handle the signal, it knows better
than us wether it wants to cleanup and terminate or do something different.

So now we pass an empty context just to make the API of interp.Runner happy

Fixes go-task/task/#458

go-task/task,go-task/task