./miri run: verbose by default, add flag to be quiet

[RalfJung]

@oli-obk IIRC you mad this quiet-by-default so that it behaves like just running the program. I think that's rarely relevant... and quite often, you end up typing ./miri run and then wonder what the heck it is doing when it takes 60s before it even begins running the program (while it rebuilds Miri or the sysroot or whatever).

The is precedent for "by default, show the build progress": cargo run works that way. So I propose we make ./miri run behave more like cargo run (with a -q flag to be quiet).

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