"./miri test" builds Miri twice

[RalfJung]

We (again) have some dependency situation which leads to Miri being built twice for ./miri test -- likely one of our dev-dependencies enables a feature on another dependency that is shared with a regular build of Miri and so the Miri binary gets rebuilt by cargo.

Is there any way we can automatically test this, and avoid having to manually track this down at least once a year?

Is there some cargo bugreport that would have cargo re-use the perfectly fine miri binary that we have rather than re-building it with more features enabled that we didn't need?

[RalfJung]

For now I worked around this by always telling cargo to build everything... only ./miri run still has a separate build now.

#3372 may be a better solution.